diff options
Diffstat (limited to 'lib/htmlpurifier/docs/enduser-utf8.html')
| -rw-r--r-- | lib/htmlpurifier/docs/enduser-utf8.html | 1060 |
1 files changed, 0 insertions, 1060 deletions
diff --git a/lib/htmlpurifier/docs/enduser-utf8.html b/lib/htmlpurifier/docs/enduser-utf8.html deleted file mode 100644 index 9b01a302a..000000000 --- a/lib/htmlpurifier/docs/enduser-utf8.html +++ /dev/null @@ -1,1060 +0,0 @@ -<?xml version="1.0" encoding="UTF-8"?> -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" - "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> -<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"><head> -<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> -<meta name="description" content="Describes the rationale for using UTF-8, the ramifications otherwise, and how to make the switch." /> -<link rel="stylesheet" type="text/css" href="./style.css" /> -<style type="text/css"> - .minor td {font-style:italic;} -</style> - -<title>UTF-8: The Secret of Character Encoding - HTML Purifier</title> - -<!-- Note to users: this document, though professing to be UTF-8, attempts -to use only ASCII characters, because most webservers are configured -to send HTML as ISO-8859-1. So I will, many times, go against my -own advice for sake of portability. --> - -</head><body> - -<h1>UTF-8: The Secret of Character Encoding</h1> - -<div id="filing">Filed under End-User</div> -<div id="index">Return to the <a href="index.html">index</a>.</div> -<div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div> - -<p>Character encoding and character sets are not that -difficult to understand, but so many people blithely stumble -through the worlds of programming without knowing what to actually -do about it, or say "Ah, it's a job for those <em>internationalization</em> -experts." No, it is not! This document will walk you through -determining the encoding of your system and how you should handle -this information. It will stay away from excessive discussion on -the internals of character encoding.</p> - -<p>This document is not designed to be read in its entirety: it will -slowly introduce concepts that build on each other: you need not get to -the bottom to have learned something new. However, I strongly -recommend you read all the way to <strong>Why UTF-8?</strong>, because at least -at that point you'd have made a conscious decision not to migrate, -which can be a rewarding (but difficult) task.</p> - -<blockquote class="aside"> -<div class="label">Asides</div> - <p>Text in this formatting is an <strong>aside</strong>, - interesting tidbits for the curious but not strictly necessary material to - do the tutorial. If you read this text, you'll come out - with a greater understanding of the underlying issues.</p> -</blockquote> - -<h2>Table of Contents</h2> - -<ol id="toc"> - <li><a href="#findcharset">Finding the real encoding</a></li> - <li><a href="#findmetacharset">Finding the embedded encoding</a></li> - <li><a href="#fixcharset">Fixing the encoding</a><ol> - <li><a href="#fixcharset-none">No embedded encoding</a></li> - <li><a href="#fixcharset-diff">Embedded encoding disagrees</a></li> - <li><a href="#fixcharset-server">Changing the server encoding</a><ol> - <li><a href="#fixcharset-server-php">PHP header() function</a></li> - <li><a href="#fixcharset-server-phpini">PHP ini directive</a></li> - <li><a href="#fixcharset-server-nophp">Non-PHP</a></li> - <li><a href="#fixcharset-server-htaccess">.htaccess</a></li> - <li><a href="#fixcharset-server-ext">File extensions</a></li> - </ol></li> - <li><a href="#fixcharset-xml">XML</a></li> - <li><a href="#fixcharset-internals">Inside the process</a></li> - </ol></li> - <li><a href="#whyutf8">Why UTF-8?</a><ol> - <li><a href="#whyutf8-i18n">Internationalization</a></li> - <li><a href="#whyutf8-user">User-friendly</a></li> - <li><a href="#whyutf8-forms">Forms</a><ol> - <li><a href="#whyutf8-forms-urlencoded">application/x-www-form-urlencoded</a></li> - <li><a href="#whyutf8-forms-multipart">multipart/form-data</a></li> - </ol></li> - <li><a href="#whyutf8-support">Well supported</a></li> - <li><a href="#whyutf8-htmlpurifier">HTML Purifiers</a></li> - </ol></li> - <li><a href="#migrate">Migrate to UTF-8</a><ol> - <li><a href="#migrate-db">Configuring your database</a><ol> - <li><a href="#migrate-db-legit">Legit method</a></li> - <li><a href="#migrate-db-binary">Binary</a></li> - </ol></li> - <li><a href="#migrate-editor">Text editor</a></li> - <li><a href="#migrate-bom">Byte Order Mark (headers already sent!)</a></li> - <li><a href="#migrate-fonts">Fonts</a><ol> - <li><a href="#migrate-fonts-obscure">Obscure scripts</a></li> - <li><a href="#migrate-fonts-occasional">Occasional use</a></li> - </ol></li> - <li><a href="#migrate-variablewidth">Dealing with variable width in functions</a></li> - </ol></li> - <li><a href="#externallinks">Further Reading</a></li> -</ol> - -<h2 id="findcharset">Finding the real encoding</h2> - -<p>In the beginning, there was ASCII, and things were simple. But they -weren't good, for no one could write in Cyrillic or Thai. So there -exploded a proliferation of character encodings to remedy the problem -by extending the characters ASCII could express. This ridiculously -simplified version of the history of character encodings shows us that -there are now many character encodings floating around.</p> - -<blockquote class="aside"> - <p>A <strong>character encoding</strong> tells the computer how to - interpret raw zeroes and ones into real characters. It - usually does this by pairing numbers with characters.</p> - <p>There are many different types of character encodings floating - around, but the ones we deal most frequently with are ASCII, - 8-bit encodings, and Unicode-based encodings.</p> - <ul> - <li><strong>ASCII</strong> is a 7-bit encoding based on the - English alphabet.</li> - <li><strong>8-bit encodings</strong> are extensions to ASCII - that add a potpourri of useful, non-standard characters - like é and æ. They can only add 127 characters, - so usually only support one script at a time. When you - see a page on the web, chances are it's encoded in one - of these encodings.</li> - <li><strong>Unicode-based encodings</strong> implement the - Unicode standard and include UTF-8, UTF-16 and UTF-32/UCS-4. - They go beyond 8-bits and support almost - every language in the world. UTF-8 is gaining traction - as the dominant international encoding of the web.</li> - </ul> -</blockquote> - -<p>The first step of our journey is to find out what the encoding of -your website is. The most reliable way is to ask your -browser:</p> - -<dl> - <dt>Mozilla Firefox</dt> - <dd>Tools > Page Info: Encoding</dd> - <dt>Internet Explorer</dt> - <dd>View > Encoding: bulleted item is unofficial name</dd> -</dl> - -<p>Internet Explorer won't give you the MIME (i.e. useful/real) name of the -character encoding, so you'll have to look it up using their description. -Some common ones:</p> - -<table class="table"> - <thead><tr> - <th>IE's Description</th> - <th>Mime Name</th> - </tr></thead> - <tbody> - <tr><th colspan="2">Windows</th></tr> - <tr><td>Arabic (Windows)</td><td>Windows-1256</td></tr> - <tr><td>Baltic (Windows)</td><td>Windows-1257</td></tr> - <tr><td>Central European (Windows)</td><td>Windows-1250</td></tr> - <tr><td>Cyrillic (Windows)</td><td>Windows-1251</td></tr> - <tr><td>Greek (Windows)</td><td>Windows-1253</td></tr> - <tr><td>Hebrew (Windows)</td><td>Windows-1255</td></tr> - <tr><td>Thai (Windows)</td><td>TIS-620</td></tr> - <tr><td>Turkish (Windows)</td><td>Windows-1254</td></tr> - <tr><td>Vietnamese (Windows)</td><td>Windows-1258</td></tr> - <tr><td>Western European (Windows)</td><td>Windows-1252</td></tr> - </tbody> - <tbody> - <tr><th colspan="2">ISO</th></tr> - <tr><td>Arabic (ISO)</td><td>ISO-8859-6</td></tr> - <tr><td>Baltic (ISO)</td><td>ISO-8859-4</td></tr> - <tr><td>Central European (ISO)</td><td>ISO-8859-2</td></tr> - <tr><td>Cyrillic (ISO)</td><td>ISO-8859-5</td></tr> - <tr class="minor"><td>Estonian (ISO)</td><td>ISO-8859-13</td></tr> - <tr class="minor"><td>Greek (ISO)</td><td>ISO-8859-7</td></tr> - <tr><td>Hebrew (ISO-Logical)</td><td>ISO-8859-8-l</td></tr> - <tr><td>Hebrew (ISO-Visual)</td><td>ISO-8859-8</td></tr> - <tr class="minor"><td>Latin 9 (ISO)</td><td>ISO-8859-15</td></tr> - <tr class="minor"><td>Turkish (ISO)</td><td>ISO-8859-9</td></tr> - <tr><td>Western European (ISO)</td><td>ISO-8859-1</td></tr> - </tbody> - <tbody> - <tr><th colspan="2">Other</th></tr> - <tr><td>Chinese Simplified (GB18030)</td><td>GB18030</td></tr> - <tr><td>Chinese Simplified (GB2312)</td><td>GB2312</td></tr> - <tr><td>Chinese Simplified (HZ)</td><td>HZ</td></tr> - <tr><td>Chinese Traditional (Big5)</td><td>Big5</td></tr> - <tr><td>Japanese (Shift-JIS)</td><td>Shift_JIS</td></tr> - <tr><td>Japanese (EUC)</td><td>EUC-JP</td></tr> - <tr><td>Korean</td><td>EUC-KR</td></tr> - <tr><td>Unicode (UTF-8)</td><td>UTF-8</td></tr> - </tbody> -</table> - -<p>Internet Explorer does not recognize some of the more obscure -character encodings, and having to lookup the real names with a table -is a pain, so I recommend using Mozilla Firefox to find out your -character encoding.</p> - -<h2 id="findmetacharset">Finding the embedded encoding</h2> - -<p>At this point, you may be asking, "Didn't we already find out our -encoding?" Well, as it turns out, there are multiple places where -a web developer can specify a character encoding, and one such place -is in a <code>META</code> tag:</p> - -<pre><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /></pre> - -<p>You'll find this in the <code>HEAD</code> section of an HTML document. -The text to the right of <code>charset=</code> is the "claimed" -encoding: the HTML claims to be this encoding, but whether or not this -is actually the case depends on other factors. For now, take note -if your <code>META</code> tag claims that either:</p> - -<ol> - <li>The character encoding is the same as the one reported by the - browser,</li> - <li>The character encoding is different from the browser's, or</li> - <li>There is no <code>META</code> tag at all! (horror, horror!)</li> -</ol> - -<h2 id="fixcharset">Fixing the encoding</h2> - -<p class="aside">The advice given here is for pages being served as -vanilla <code>text/html</code>. Different practices must be used -for <code>application/xml</code> or <code>application/xml+xhtml</code>, see -<a href="http://www.w3.org/TR/2002/NOTE-xhtml-media-types-20020430/">W3C's -document on XHTML media types</a> for more information.</p> - -<p>If your <code>META</code> encoding and your real encoding match, -savvy! You can skip this section. If they don't...</p> - -<h3 id="fixcharset-none">No embedded encoding</h3> - -<p>If this is the case, you'll want to add in the appropriate -<code>META</code> tag to your website. It's as simple as copy-pasting -the code snippet above and replacing UTF-8 with whatever is the mime name -of your real encoding.</p> - -<blockquote class="aside"> - <p>For all those skeptics out there, there is a very good reason - why the character encoding should be explicitly stated. When the - browser isn't told what the character encoding of a text is, it - has to guess: and sometimes the guess is wrong. Hackers can manipulate - this guess in order to slip XSS past filters and then fool the - browser into executing it as active code. A great example of this - is the <a href="http://shiflett.org/archive/177">Google UTF-7 - exploit</a>.</p> - <p>You might be able to get away with not specifying a character - encoding with the <code>META</code> tag as long as your webserver - sends the right Content-Type header, but why risk it? Besides, if - the user downloads the HTML file, there is no longer any webserver - to define the character encoding.</p> -</blockquote> - -<h3 id="fixcharset-diff">Embedded encoding disagrees</h3> - -<p>This is an extremely common mistake: another source is telling -the browser what the -character encoding is and is overriding the embedded encoding. This -source usually is the Content-Type HTTP header that the webserver (i.e. -Apache) sends. A usual Content-Type header sent with a page might -look like this:</p> - -<pre>Content-Type: text/html; charset=ISO-8859-1</pre> - -<p>Notice how there is a charset parameter: this is the webserver's -way of telling a browser what the character encoding is, much like -the <code>META</code> tags we touched upon previously.</p> - -<blockquote class="aside"><p>In fact, the <code>META</code> tag is -designed as a substitute for the HTTP header for contexts where -sending headers is impossible (such as locally stored files without -a webserver). Thus the name <code>http-equiv</code> (HTTP equivalent). -</p></blockquote> - -<p>There are two ways to go about fixing this: changing the <code>META</code> -tag to match the HTTP header, or changing the HTTP header to match -the <code>META</code> tag. How do we know which to do? It depends -on the website's content: after all, headers and tags are only ways of -describing the actual characters on the web page.</p> - -<p>If your website:</p> - -<dl> - <dt>...only uses ASCII characters,</dt> - <dd>Either way is fine, but I recommend switching both to - UTF-8 (more on this later).</dd> - <dt>...uses special characters, and they display - properly,</dt> - <dd>Change the embedded encoding to the server encoding.</dd> - <dt>...uses special characters, but users often complain that - they come out garbled,</dt> - <dd>Change the server encoding to the embedded encoding.</dd> -</dl> - -<p>Changing a META tag is easy: just swap out the old encoding -for the new. Changing the server (HTTP header) encoding, however, -is slightly more difficult.</p> - -<h3 id="fixcharset-server">Changing the server encoding</h3> - -<h4 id="fixcharset-server-php">PHP header() function</h4> - -<p>The simplest way to handle this problem is to send the encoding -yourself, via your programming language. Since you're using HTML -Purifier, I'll assume PHP, although it's not too difficult to do -similar things in -<a href="http://www.w3.org/International/O-HTTP-charset#scripting">other -languages</a>. The appropriate code is:</p> - -<pre><a href="http://php.net/function.header">header</a>('Content-Type:text/html; charset=UTF-8');</pre> - -<p>...replacing UTF-8 with whatever your embedded encoding is. -This code must come before any output, so be careful about -stray whitespace in your application (i.e., any whitespace before -output excluding whitespace within <?php ?> tags).</p> - -<h4 id="fixcharset-server-phpini">PHP ini directive</h4> - -<p>PHP also has a neat little ini directive that can save you a -header call: <code><a href="http://php.net/ini.core#ini.default-charset">default_charset</a></code>. Using this code:</p> - -<pre><a href="http://php.net/function.ini_set">ini_set</a>('default_charset', 'UTF-8');</pre> - -<p>...will also do the trick. If PHP is running as an Apache module (and -not as FastCGI, consult -<a href="http://php.net/phpinfo">phpinfo</a>() for details), you can even use htaccess to apply this property -across many PHP files:</p> - -<pre><a href="http://php.net/configuration.changes#configuration.changes.apache">php_value</a> default_charset "UTF-8"</pre> - -<blockquote class="aside"><p>As with all INI directives, this can -also go in your php.ini file. Some hosting providers allow you to customize -your own php.ini file, ask your support for details. Use:</p> -<pre>default_charset = "utf-8"</pre></blockquote> - -<h4 id="fixcharset-server-nophp">Non-PHP</h4> - -<p>You may, for whatever reason, need to set the character encoding -on non-PHP files, usually plain ol' HTML files. Doing this -is more of a hit-or-miss process: depending on the software being -used as a webserver and the configuration of that software, certain -techniques may work, or may not work.</p> - -<h4 id="fixcharset-server-htaccess">.htaccess</h4> - -<p>On Apache, you can use an .htaccess file to change the character -encoding. I'll defer to -<a href="http://www.w3.org/International/questions/qa-htaccess-charset">W3C</a> -for the in-depth explanation, but it boils down to creating a file -named .htaccess with the contents:</p> - -<pre><a href="http://httpd.apache.org/docs/1.3/mod/mod_mime.html#addcharset">AddCharset</a> UTF-8 .html</pre> - -<p>Where UTF-8 is replaced with the character encoding you want to -use and .html is a file extension that this will be applied to. This -character encoding will then be set for any file directly in -or in the subdirectories of directory you place this file in.</p> - -<p>If you're feeling particularly courageous, you can use:</p> - -<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> UTF-8</pre> - -<p>...which changes the character set Apache adds to any document that -doesn't have any Content-Type parameters. This directive, which the -default configuration file sets to iso-8859-1 for security -reasons, is probably why your headers mismatch -with the <code>META</code> tag. If you would prefer Apache not to be -butting in on your character encodings, you can tell it not -to send anything at all:</p> - -<pre><a href="http://httpd.apache.org/docs/1.3/mod/core.html#adddefaultcharset">AddDefaultCharset</a> Off</pre> - -<p>...making your internal charset declaration (usually the <code>META</code> tags) -the sole source of character encoding -information. In these cases, it is <em>especially</em> important to make -sure you have valid <code>META</code> tags on your pages and all the -text before them is ASCII.</p> - -<blockquote class="aside"><p>These directives can also be -placed in httpd.conf file for Apache, but -in most shared hosting situations you won't be able to edit this file. -</p></blockquote> - -<h4 id="fixcharset-server-ext">File extensions</h4> - -<p>If you're not allowed to use .htaccess files, you can often -piggy-back off of Apache's default AddCharset declarations to get -your files in the proper extension. Here are Apache's default -character set declarations:</p> - -<table class="table"> - <thead><tr> - <th>Charset</th> - <th>File extension(s)</th> - </tr></thead> - <tbody> - <tr><td>ISO-8859-1</td><td>.iso8859-1 .latin1</td></tr> - <tr><td>ISO-8859-2</td><td>.iso8859-2 .latin2 .cen</td></tr> - <tr><td>ISO-8859-3</td><td>.iso8859-3 .latin3</td></tr> - <tr><td>ISO-8859-4</td><td>.iso8859-4 .latin4</td></tr> - <tr><td>ISO-8859-5</td><td>.iso8859-5 .latin5 .cyr .iso-ru</td></tr> - <tr><td>ISO-8859-6</td><td>.iso8859-6 .latin6 .arb</td></tr> - <tr><td>ISO-8859-7</td><td>.iso8859-7 .latin7 .grk</td></tr> - <tr><td>ISO-8859-8</td><td>.iso8859-8 .latin8 .heb</td></tr> - <tr><td>ISO-8859-9</td><td>.iso8859-9 .latin9 .trk</td></tr> - <tr><td>ISO-2022-JP</td><td>.iso2022-jp .jis</td></tr> - <tr><td>ISO-2022-KR</td><td>.iso2022-kr .kis</td></tr> - <tr><td>ISO-2022-CN</td><td>.iso2022-cn .cis</td></tr> - <tr><td>Big5</td><td>.Big5 .big5 .b5</td></tr> - <tr><td>WINDOWS-1251</td><td>.cp-1251 .win-1251</td></tr> - <tr><td>CP866</td><td>.cp866</td></tr> - <tr><td>KOI8-r</td><td>.koi8-r .koi8-ru</td></tr> - <tr><td>KOI8-ru</td><td>.koi8-uk .ua</td></tr> - <tr><td>ISO-10646-UCS-2</td><td>.ucs2</td></tr> - <tr><td>ISO-10646-UCS-4</td><td>.ucs4</td></tr> - <tr><td>UTF-8</td><td>.utf8</td></tr> - <tr><td>GB2312</td><td>.gb2312 .gb </td></tr> - <tr><td>utf-7</td><td>.utf7</td></tr> - <tr><td>EUC-TW</td><td>.euc-tw</td></tr> - <tr><td>EUC-JP</td><td>.euc-jp</td></tr> - <tr><td>EUC-KR</td><td>.euc-kr</td></tr> - <tr><td>shift_jis</td><td>.sjis</td></tr> - </tbody> -</table> - -<p>So, for example, a file named <code>page.utf8.html</code> or -<code>page.html.utf8</code> will probably be sent with the UTF-8 charset -attached, the difference being that if there is an -<code>AddCharset charset .html</code> declaration, it will override -the .utf8 extension in <code>page.utf8.html</code> (precedence moves -from right to left). By default, Apache has no such declaration.</p> - -<h4 id="fixcharset-server-iis">Microsoft IIS</h4> - -<p>If anyone can contribute information on how to configure Microsoft -IIS to change character encodings, I'd be grateful.</p> - -<h3 id="fixcharset-xml">XML</h3> - -<p><code>META</code> tags are the most common source of embedded -encodings, but they can also come from somewhere else: XML -Declarations. They look like:</p> - -<pre><?xml version="1.0" encoding="UTF-8"?></pre> - -<p>...and are most often found in XML documents (including XHTML).</p> - -<p>For XHTML, this XML Declaration theoretically -overrides the <code>META</code> tag. In reality, this happens only when the -XHTML is actually served as legit XML and not HTML, which is almost always -never due to Internet Explorer's lack of support for -<code>application/xhtml+xml</code> (even though doing so is often -argued to be <a href="http://www.hixie.ch/advocacy/xhtml">good -practice</a> and is required by the XHTML 1.1 specification).</p> - -<p>For XML, however, this XML Declaration is extremely important. -Since most webservers are not configured to send charsets for .xml files, -this is the only thing a parser has to go on. Furthermore, the default -for XML files is UTF-8, which often butts heads with more common -ISO-8859-1 encoding (you see this in garbled RSS feeds).</p> - -<p>In short, if you use XHTML and have gone through the -trouble of adding the XML Declaration, make sure it jives -with your <code>META</code> tags (which should only be present -if served in text/html) and HTTP headers.</p> - -<h3 id="fixcharset-internals">Inside the process</h3> - -<p>This section is not required reading, -but may answer some of your questions on what's going on in all -this character encoding hocus pocus. If you're interested in -moving on to the next phase, skip this section.</p> - -<p>A logical question that follows all of our wheeling and dealing -with multiple sources of character encodings is "Why are there -so many options?" To answer this question, we have to turn -back our definition of character encodings: they allow a program -to interpret bytes into human-readable characters.</p> - -<p>Thus, a chicken-egg problem: a character encoding -is necessary to interpret the -text of a document. A <code>META</code> tag is in the text of a document. -The <code>META</code> tag gives the character encoding. How can we -determine the contents of a <code>META</code> tag, inside the text, -if we don't know it's character encoding? And how do we figure out -the character encoding, if we don't know the contents of the -<code>META</code> tag?</p> - -<p>Fortunately for us, the characters we need to write the -<code>META</code> are in ASCII, which is pretty much universal -over every character encoding that is in common use today. So, -all the web-browser has to do is parse all the way down until -it gets to the Content-Type tag, extract the character encoding -tag, then re-parse the document according to this new information.</p> - -<p>Obviously this is complicated, so browsers prefer the simpler -and more efficient solution: get the character encoding from a -somewhere other than the document itself, i.e. the HTTP headers, -much to the chagrin of HTML authors who can't set these headers.</p> - -<h2 id="whyutf8">Why UTF-8?</h2> - -<p>So, you've gone through all the trouble of ensuring that your -server and embedded characters all line up properly and are -present. Good job: at -this point, you could quit and rest easy knowing that your pages -are not vulnerable to character encoding style XSS attacks. -However, just as having a character encoding is better than -having no character encoding at all, having UTF-8 as your -character encoding is better than having some other random -character encoding, and the next step is to convert to UTF-8. -But why?</p> - -<h3 id="whyutf8-i18n">Internationalization</h3> - -<p>Many software projects, at one point or another, suddenly realize -that they should be supporting more than one language. Even regular -usage in one language sometimes requires the occasional special character -that, without surprise, is not available in your character set. Sometimes -developers get around this by adding support for multiple encodings: when -using Chinese, use Big5, when using Japanese, use Shift-JIS, when -using Greek, etc. Other times, they use character references with great -zeal.</p> - -<p>UTF-8, however, obviates the need for any of these complicated -measures. After getting the system to use UTF-8 and adjusting for -sources that are outside the hand of the browser (more on this later), -UTF-8 just works. You can use it for any language, even many languages -at once, you don't have to worry about managing multiple encodings, -you don't have to use those user-unfriendly entities.</p> - -<h3 id="whyutf8-user">User-friendly</h3> - -<p>Websites encoded in Latin-1 (ISO-8859-1) which occasionally need -a special character outside of their scope often will use a character -entity reference to achieve the desired effect. For instance, θ can be -written <code>&theta;</code>, regardless of the character encoding's -support of Greek letters.</p> - -<p>This works nicely for limited use of special characters, but -say you wanted this sentence of Chinese text: 激光, -這兩個字是甚麼意思. -The ampersand encoded version would look like this:</p> - -<pre>&#28608;&#20809;, &#36889;&#20841;&#20491;&#23383;&#26159;&#29978;&#40636;&#24847;&#24605;</pre> - -<p>Extremely inconvenient for those of us who actually know what -character entities are, totally unintelligible to poor users who don't! -Even the slightly more user-friendly, "intelligible" character -entities like <code>&theta;</code> will leave users who are -uninterested in learning HTML scratching their heads. On the other -hand, if they see θ in an edit box, they'll know that it's a -special character, and treat it accordingly, even if they don't know -how to write that character themselves.</p> - -<blockquote class="aside"><p>Wikipedia is a great case study for -an application that originally used ISO-8859-1 but switched to UTF-8 -when it became far to cumbersome to support foreign languages. Bots -will now actually go through articles and convert character entities -to their corresponding real characters for the sake of user-friendliness -and searchability. See -<a href="http://meta.wikimedia.org/wiki/Help:Special_characters">Meta's -page on special characters</a> for more details. -</p></blockquote> - -<h3 id="whyutf8-forms">Forms</h3> - -<p>While we're on the tack of users, how do non-UTF-8 web forms deal -with characters that are outside of their character set? Rather than -discuss what UTF-8 does right, we're going to show what could go wrong -if you didn't use UTF-8 and people tried to use characters outside -of your character encoding.</p> - -<p>The troubles are large, extensive, and extremely difficult to fix (or, -at least, difficult enough that if you had the time and resources to invest -in doing the fix, you would be probably better off migrating to UTF-8). -There are two types of form submission: <code>application/x-www-form-urlencoded</code> -which is used for GET and by default for POST, and <code>multipart/form-data</code> -which may be used by POST, and is required when you want to upload -files.</p> - -<p>The following is a summarization of notes from -<a href="http://web.archive.org/web/20060427015200/ppewww.ph.gla.ac.uk/~flavell/charset/form-i18n.html"> -<code>FORM</code> submission and i18n</a>. That document contains lots -of useful information, but is written in a rambly manner, so -here I try to get right to the point. (Note: the original has -disappeared off the web, so I am linking to the Web Archive copy.)</p> - -<h4 id="whyutf8-forms-urlencoded"><code>application/x-www-form-urlencoded</code></h4> - -<p>This is the Content-Type that GET requests must use, and POST requests -use by default. It involves the ubiquitous percent encoding format that -looks something like: <code>%C3%86</code>. There is no official way of -determining the character encoding of such a request, since the percent -encoding operates on a byte level, so it is usually assumed that it -is the same as the encoding the page containing the form was submitted -in. (<a href="http://tools.ietf.org/html/rfc3986#section-2.5">RFC 3986</a> -recommends that textual identifiers be translated to UTF-8; however, browser -compliance is spotty.) You'll run into very few problems -if you only use characters in the character encoding you chose.</p> - -<p>However, once you start adding characters outside of your encoding -(and this is a lot more common than you may think: take curly -"smart" quotes from Microsoft as an example), -a whole manner of strange things start to happen. Depending on the -browser you're using, they might:</p> - -<ul> - <li>Replace the unsupported characters with useless question marks,</li> - <li>Attempt to fix the characters (example: smart quotes to regular quotes),</li> - <li>Replace the character with a character entity reference, or</li> - <li>Send it anyway as a different character encoding mixed in - with the original encoding (usually Windows-1252 rather than - iso-8859-1 or UTF-8 interspersed in 8-bit)</li> -</ul> - -<p>To properly guard against these behaviors, you'd have to sniff out -the browser agent, compile a database of different behaviors, and -take appropriate conversion action against the string (disregarding -a spate of extremely mysterious, random and devastating bugs Internet -Explorer manifests every once in a while). Or you could -use UTF-8 and rest easy knowing that none of this could possibly happen -since UTF-8 supports every character.</p> - -<h4 id="whyutf8-forms-multipart"><code>multipart/form-data</code></h4> - -<p>Multipart form submission takes away a lot of the ambiguity -that percent-encoding had: the server now can explicitly ask for -certain encodings, and the client can explicitly tell the server -during the form submission what encoding the fields are in.</p> - -<p>There are two ways you go with this functionality: leave it -unset and have the browser send in the same encoding as the page, -or set it to UTF-8 and then do another conversion server-side. -Each method has deficiencies, especially the former.</p> - -<p>If you tell the browser to send the form in the same encoding as -the page, you still have the trouble of what to do with characters -that are outside of the character encoding's range. The behavior, once -again, varies: Firefox 2.0 converts them to character entity references -while Internet Explorer 7.0 mangles them beyond intelligibility. For -serious internationalization purposes, this is not an option.</p> - -<p>The other possibility is to set Accept-Encoding to UTF-8, which -begs the question: Why aren't you using UTF-8 for everything then? -This route is more palatable, but there's a notable caveat: your data -will come in as UTF-8, so you will have to explicitly convert it into -your favored local character encoding.</p> - -<p>I object to this approach on idealogical grounds: you're -digging yourself deeper into -the hole when you could have been converting to UTF-8 -instead. And, of course, you can't use this method for GET requests.</p> - -<h3 id="whyutf8-support">Well supported</h3> - -<p>Almost every modern browser in the wild today has full UTF-8 and Unicode -support: the number of troublesome cases can be counted with the -fingers of one hand, and these browsers usually have trouble with -other character encodings too. Problems users usually encounter stem -from the lack of appropriate fonts to display the characters (once -again, this applies to all character encodings and HTML entities) or -Internet Explorer's lack of intelligent font picking (which can be -worked around).</p> - -<p>We will go into more detail about how to deal with edge cases in -the browser world in the Migration section, but rest assured that -converting to UTF-8, if done correctly, will not result in users -hounding you about broken pages.</p> - -<h3 id="whyutf8-htmlpurifier">HTML Purifier</h3> - -<p>And finally, we get to HTML Purifier. HTML Purifier is built to -deal with UTF-8: any indications otherwise are the result of an -encoder that converts text from your preferred encoding to UTF-8, and -back again. HTML Purifier never touches anything else, and leaves -it up to the module iconv to do the dirty work.</p> - -<p>This approach, however, is not perfect. iconv is blithely unaware -of HTML character entities. HTML Purifier, in order to -protect against sophisticated escaping schemes, normalizes all character -and numeric entity references before processing the text. This leads to -one important ramification:</p> - -<p><strong>Any character that is not supported by the target character -set, regardless of whether or not it is in the form of a character -entity reference or a raw character, will be silently ignored.</strong></p> - -<p>Example of this principle at work: say you have <code>&theta;</code> -in your HTML, but the output is in Latin-1 (which, understandably, -does not understand Greek), the following process will occur (assuming you've -set the encoding correctly using %Core.Encoding):</p> - -<ul> - <li>The <code>Encoder</code> will transform the text from ISO 8859-1 to UTF-8 - (note that theta is preserved here since it doesn't actually use - any non-ASCII characters): <code>&theta;</code></li> - <li>The <code>EntityParser</code> will transform all named and numeric - character entities to their corresponding raw UTF-8 equivalents: - <code>θ</code></li> - <li>HTML Purifier processes the code: <code>θ</code></li> - <li>The <code>Encoder</code> now transforms the text back from UTF-8 - to ISO 8859-1. Since Greek is not supported by ISO 8859-1, it - will be either ignored or replaced with a question mark: - <code>?</code></li> -</ul> - -<p>This behaviour is quite unsatisfactory. It is a deal-breaker for -international applications, and it can be mildly annoying for the provincial -soul who occasionally needs a special character. Since 1.4.0, HTML -Purifier has provided a slightly more palatable workaround using -%Core.EscapeNonASCIICharacters. The process now looks like:</p> - -<ul> - <li>The <code>Encoder</code> transforms encoding to UTF-8: <code>&theta;</code></li> - <li>The <code>EntityParser</code> transforms entities: <code>θ</code></li> - <li>HTML Purifier processes the code: <code>θ</code></li> - <li>The <code>Encoder</code> replaces all non-ASCII characters - with numeric entity reference: <code>&#952;</code></li> - <li>For good measure, <code>Encoder</code> transforms encoding back to - original (which is strictly unnecessary for 99% of encodings - out there): <code>&#952;</code> (remember, it's all ASCII!)</li> -</ul> - -<p>...which means that this is only good for an occasional foray into -the land of Unicode characters, and is totally unacceptable for Chinese -or Japanese texts. The even bigger kicker is that, supposing the -input encoding was actually ISO-8859-7, which <em>does</em> support -theta, the character would get converted into a character entity reference -anyway! (The Encoder does not discriminate).</p> - -<p>The current functionality is about where HTML Purifier will be for -the rest of eternity. HTML Purifier could attempt to preserve the original -form of the character references so that they could be substituted back in, only the -DOM extension kills them off irreversibly. HTML Purifier could also attempt -to be smart and only convert non-ASCII characters that weren't supported -by the target encoding, but that would require reimplementing iconv -with HTML awareness, something I will not do.</p> - -<p>So there: either it's UTF-8 or crippled international support. Your pick! (and I'm -not being sarcastic here: some people could care less about other languages).</p> - -<h2 id="migrate">Migrate to UTF-8</h2> - -<p>So, you've decided to bite the bullet, and want to migrate to UTF-8. -Note that this is not for the faint-hearted, and you should expect -the process to take longer than you think it will take.</p> - -<p>The general idea is that you convert all existing text to UTF-8, -and then you set all the headers and META tags we discussed earlier -to UTF-8. There are many ways going about doing this: you could -write a conversion script that runs through the database and re-encodes -everything as UTF-8 or you could do the conversion on the fly when someone -reads the page. The details depend on your system, but I will cover -some of the more subtle points of migration that may trip you up.</p> - -<h3 id="migrate-db">Configuring your database</h3> - -<p>Most modern databases, the most prominent open-source ones being MySQL -4.1+ and PostgreSQL, support character encodings. If you're switching -to UTF-8, logically speaking, you'd want to make sure your database -knows about the change too. There are some caveats though:</p> - -<h4 id="migrate-db-legit">Legit method</h4> - -<p>Standardization in terms of SQL syntax for specifying character -encodings is notoriously spotty. Refer to your respective database's -documentation on how to do this properly.</p> - -<p>For <a href="http://dev.mysql.com/doc/refman/5.0/en/charset-conversion.html">MySQL</a>, <code>ALTER</code> will magically perform the -character encoding conversion for you. However, you have -to make sure that the text inside the column is what is says it is: -if you had put Shift-JIS in an ISO 8859-1 column, MySQL will irreversibly mangle -the text when you try to convert it to UTF-8. You'll have to convert -it to a binary field, convert it to a Shift-JIS field (the real encoding), -and then finally to UTF-8. Many a website had pages irreversibly mangled -because they didn't realize that they'd been deluding themselves about -the character encoding all along; don't become the next victim.</p> - -<p>For <a href="http://www.postgresql.org/docs/8.2/static/multibyte.html">PostgreSQL</a>, there appears to be no direct way to change the -encoding of a database (as of 8.2). You will have to dump the data, and then reimport -it into a new table. Make sure that your client encoding is set properly: -this is how PostgreSQL knows to perform an encoding conversion.</p> - -<p>Many times, you will be also asked about the "collation" of -the new column. Collation is how a DBMS sorts text, like ordering -B, C and A into A, B and C (the problem gets surprisingly complicated -when you get to languages like Thai and Japanese). If in doubt, -going with the default setting is usually a safe bet.</p> - -<p>Once the conversion is all said and done, you still have to remember -to set the client encoding (your encoding) properly on each database -connection using <code>SET NAMES</code> (which is standard SQL and is -usually supported).</p> - -<h4 id="migrate-db-binary">Binary</h4> - -<p>Due to the aforementioned compatibility issues, a more interoperable -way of storing UTF-8 text is to stuff it in a binary datatype. -<code>CHAR</code> becomes <code>BINARY</code>, <code>VARCHAR</code> becomes -<code>VARBINARY</code> and <code>TEXT</code> becomes <code>BLOB</code>. -Doing so can save you some huge headaches:</p> - -<ul> - <li>The syntax for binary data types is very portable,</li> - <li>MySQL 4.0 has <em>no</em> support for character encodings, so - if you want to support it you <em>have</em> to use binary,</li> - <li>MySQL, as of 5.1, has no support for four byte UTF-8 characters, - which represent characters beyond the basic multilingual - plane, and</li> - <li>You will never have to worry about your DBMS being too smart - and attempting to convert your text when you don't want it to.</li> -</ul> - -<p>MediaWiki, a very prominent international application, uses binary fields -for storing their data because of point three.</p> - -<p>There are drawbacks, of course:</p> - -<ul> - <li>Database tools like PHPMyAdmin won't be able to offer you inline - text editing, since it is declared as binary,</li> - <li>It's not semantically correct: it's really text not binary - (lying to the database),</li> - <li>Unless you use the not-very-portable wizardry mentioned above, - you have to change the encoding yourself (usually, you'd do - it on the fly), and</li> - <li>You will not have collation.</li> -</ul> - -<p>Choose based on your circumstances.</p> - -<h3 id="migrate-editor">Text editor</h3> - -<p>For more flat-file oriented systems, you will often be tasked with -converting reams of existing text and HTML files into UTF-8, as well as -making sure that all new files uploaded are properly encoded. Once again, -I can only point vaguely in the right direction for converting your -existing files: make sure you backup, make sure you use -<a href="http://php.net/ref.iconv">iconv</a>(), and -make sure you know what the original character encoding of the files -is (or are, depending on the tidiness of your system).</p> - -<p>However, I can proffer more specific advice on the subject of -text editors. Many text editors have notoriously spotty Unicode support. -To find out how your editor is doing, you can check out <a -href="http://www.alanwood.net/unicode/utilities_editors.html">this list</a> -or <a href="http://en.wikipedia.org/wiki/Comparison_of_text_editors#Encoding_support">Wikipedia's list.</a> -I personally use Notepad++, which works like a charm when it comes to UTF-8. -Usually, you will have to <strong>explicitly</strong> tell the editor through some dialogue -(usually Save as or Format) what encoding you want it to use. An editor -will often offer "Unicode" as a method of saving, which is -ambiguous. Make sure you know whether or not they really mean UTF-8 -or UTF-16 (which is another flavor of Unicode).</p> - -<p>The two things to look out for are whether or not the editor -supports <strong>font mixing</strong> (multiple -fonts in one document) and whether or not it adds a <strong>BOM</strong>. -Font mixing is important because fonts rarely have support for every -language known to mankind: in order to be flexible, an editor must -be able to take a little from here and a little from there, otherwise -all your Chinese characters will come as nice boxes. We'll discuss -BOM below.</p> - -<h3 id="migrate-bom">Byte Order Mark (headers already sent!)</h3> - -<p>The BOM, or <a href="http://en.wikipedia.org/wiki/Byte_Order_Mark">Byte -Order Mark</a>, is a magical, invisible character placed at -the beginning of UTF-8 files to tell people what the encoding is and -what the endianness of the text is. It is also unnecessary.</p> - -<p>Because it's invisible, it often -catches people by surprise when it starts doing things it shouldn't -be doing. For example, this PHP file:</p> - -<pre><strong>BOM</strong><?php -header('Location: index.php'); -?></pre> - -<p>...will fail with the all too familiar <strong>Headers already sent</strong> -PHP error. And because the BOM is invisible, this culprit will go unnoticed. -My suggestion is to only use ASCII in PHP pages, but if you must, make -sure the page is saved WITHOUT the BOM.</p> - -<blockquote class="aside"> - <p>The headers the error is referring to are <strong>HTTP headers</strong>, - which are sent to the browser before any HTML to tell it various - information. The moment any regular text (and yes, a BOM counts as - ordinary text) is output, the headers must be sent, and you are - not allowed to send anymore. Thus, the error.</p> -</blockquote> - -<p>If you are reading in text files to insert into the middle of another -page, it is strongly advised (but not strictly necessary) that you replace out the UTF-8 byte -sequence for BOM <code>"\xEF\xBB\xBF"</code> before inserting it in, -via:</p> - -<pre>$text = str_replace("\xEF\xBB\xBF", '', $text);</pre> - -<h3 id="migrate-fonts">Fonts</h3> - -<p>Generally speaking, people who are having trouble with fonts fall -into two categories:</p> - -<ul> -<li>Those who want to -use an extremely obscure language for which there is very little -support even among native speakers of the language, and</li> -<li>Those where the primary language of the text is -well-supported but there are occasional characters -that aren't supported.</li> -</ul> - -<p>Yes, there's always a chance where an English user happens across -a Sinhalese website and doesn't have the right font. But an English user -who happens not to have the right fonts probably has no business reading Sinhalese -anyway. So we'll deal with the other two edge cases.</p> - -<h4 id="migrate-fonts-obscure">Obscure scripts</h4> - -<p>If you run a Bengali website, you may get comments from users who -would like to read your website but get heaps of question marks or -other meaningless characters. Fixing this problem requires the -installation of a font or language pack which is often highly -dependent on what the language is. <a href="http://bn.wikipedia.org/wiki/%E0%A6%89%E0%A6%87%E0%A6%95%E0%A6%BF%E0%A6%AA%E0%A7%87%E0%A6%A1%E0%A6%BF%E0%A6%AF%E0%A6%BC%E0%A6%BE:Bangla_script_display_and_input_help">Here is an example</a> -of such a help file for the Bengali language; I am sure there are -others out there too. You just have to point users to the appropriate -help file.</p> - -<h4 id="migrate-fonts-occasional">Occasional use</h4> - -<p>A prime example of when you'll see some very obscure Unicode -characters embedded in what otherwise would be very bland ASCII are -letters of the -<a href="http://en.wikipedia.org/wiki/International_Phonetic_Alphabet">International -Phonetic Alphabet (IPA)</a>, use to designate pronunciations in a very standard -manner (you probably see them all the time in your dictionary). Your -average font probably won't have support for all of the IPA characters -like ʘ (bilabial click) or ʒ (voiced postalveolar fricative). -So what's a poor browser to do? Font mix! Smart browsers like Mozilla Firefox -and Internet Explorer 7 will borrow glyphs from other fonts in order -to make sure that all the characters display properly.</p> - -<p>But what happens when the browser isn't smart and happens to be the -most widely used browser in the entire world? Microsoft IE 6 -is not smart enough to borrow from other fonts when a character isn't -present, so more often than not you'll be slapped with a nice big �. -To get things to work, MSIE 6 needs a little nudge. You could configure it -to use a different font to render the text, but you can achieve the same -effect by selectively changing the font for blocks of special characters -to known good Unicode fonts.</p> - -<p>Fortunately, the folks over at Wikipedia have already done all the -heavy lifting for you. Get the CSS from the horses mouth here: -<a href="http://en.wikipedia.org/wiki/MediaWiki:Common.css">Common.css</a>, -and search for ".IPA" There are also a smattering of -other classes you can use for other purposes, check out -<a href="http://meta.wikimedia.org/wiki/Help:Special_characters#Displaying_Special_Characters">this page</a> -for more details. For you lazy ones, this should work:</p> - -<pre>.Unicode { - font-family: Code2000, "TITUS Cyberbit Basic", "Doulos SIL", - "Chrysanthi Unicode", "Bitstream Cyberbit", - "Bitstream CyberBase", Thryomanes, Gentium, GentiumAlt, - "Lucida Grande", "Arial Unicode MS", "Microsoft Sans Serif", - "Lucida Sans Unicode"; - font-family /**/:inherit; /* resets fonts for everyone but IE6 */ -}</pre> - -<p>The standard usage goes along the lines of <code><span class="Unicode">Crazy -Unicode stuff here</span></code>. Characters in the -<a href="http://en.wikipedia.org/wiki/Windows_Glyph_List_4">Windows Glyph List</a> -usually don't need to be fixed, but for anything else you probably -want to play it safe. Unless, of course, you don't care about IE6 -users.</p> - -<h3 id="migrate-variablewidth">Dealing with variable width in functions</h3> - -<p>When people claim that PHP6 will solve all our Unicode problems, they're -misinformed. It will not fix any of the aforementioned troubles. It will, -however, fix the problem we are about to discuss: processing UTF-8 text -in PHP.</p> - -<p>PHP (as of PHP5) is blithely unaware of the existence of UTF-8 (with a few -notable exceptions). Sometimes, this will cause problems, other times, -this won't. So far, we've avoided discussing the architecture of -UTF-8, so, we must first ask, what is UTF-8? Yes, it supports Unicode, -and yes, it is variable width. Other traits:</p> - -<ul> - <li>Every character's byte sequence is unique and will never be found - inside the byte sequence of another character,</li> - <li>UTF-8 may use up to four bytes to encode a character,</li> - <li>UTF-8 text must be checked for well-formedness,</li> - <li>Pure ASCII is also valid UTF-8, and</li> - <li>Binary sorting will sort UTF-8 in the same order as Unicode.</li> -</ul> - -<p>Each of these traits affect different domains of text processing -in different ways. It is beyond the scope of this document to explain -what precisely these implications are. PHPWact provides -a very good <a href="http://www.phpwact.org/php/i18n/utf-8">reference document</a> -on what to expect from each function, although coverage is spotty in -some areas. Their more general notes on -<a href="http://www.phpwact.org/php/i18n/charsets">character sets</a> -are also worth looking at for information on UTF-8. Some rules of thumb -when dealing with Unicode text:</p> - -<ul> - <li>Do not EVER use functions that:<ul> - <li>...convert case (strtolower, strtoupper, ucfirst, ucwords)</li> - <li>...claim to be case-insensitive (str_ireplace, stristr, strcasecmp)</li> - </ul></li> - <li>Think twice before using functions that:<ul> - <li>...count characters (strlen will return bytes, not characters; - str_split and word_wrap may corrupt)</li> - <li>...convert characters to entity references (UTF-8 doesn't need entities)</li> - <li>...do very complex string processing (*printf)</li> - </ul></li> -</ul> - -<p>Note: this list applies to UTF-8 encoded text only: if you have -a string that you are 100% sure is ASCII, be my guest and use -<code>strtolower</code> (HTML Purifier uses this function.)</p> - -<p>Regardless, always think in bytes, not characters. If you use strpos() -to find the position of a character, it will be in bytes, but this -usually won't matter since substr() also operates with byte indices!</p> - -<p>You'll also need to make sure your UTF-8 is well-formed and will -probably need replacements for some of these functions. I recommend -using Harry Fuecks' <a href="http://phputf8.sourceforge.net/">PHP -UTF-8</a> library, rather than use mb_string directly. HTML Purifier -also defines a few useful UTF-8 compatible functions: check out -<code>Encoder.php</code> in the <code>/library/HTMLPurifier/</code> -directory.</p> - -<h2 id="externallinks">Further Reading</h2> - -<p>Well, that's it. Hopefully this document has served as a very -practical springboard into knowledge of how UTF-8 works. You may have -decided that you don't want to migrate yet: that's fine, just know -what will happen to your output and what bug reports you may receive.</p> - -<p>Many other developers have already discussed the subject of Unicode, -UTF-8 and internationalization, and I would like to defer to them for -a more in-depth look into character sets and encodings.</p> - -<ul> - <li><a href="http://www.joelonsoftware.com/articles/Unicode.html"> - The Absolute Minimum Every Software Developer Absolutely, - Positively Must Know About Unicode and Character Sets - (No Excuses!)</a> by Joel Spolsky, provides a <em>very</em> - good high-level look at Unicode and character sets in general.</li> - <li><a href="http://en.wikipedia.org/wiki/UTF-8">UTF-8 on Wikipedia</a>, - provides a lot of useful details into the innards of UTF-8, although - it may be a little off-putting to people who don't know much - about Unicode to begin with.</li> -</ul> - -</body> -</html> - -<!-- vim: et sw=4 sts=4 ---> |