docs/enduser-uri-filter.html

   1 <?xml version="1.0" encoding="UTF-8"?>
   2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
   3     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
   4 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head>
   5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
   6 <meta name="description" content="Tutorial for creating custom URI filters." />
   7 <link rel="stylesheet" type="text/css" href="style.css" />
   8
   9 <title>URI Filters - HTML Purifier</title>
  10
  11 </head><body>
  12
  13 <h1>URI Filters</h1>
  14
  15 <div id="filing">Filed under End-User</div>
  16 <div id="index">Return to the <a href="index.html">index</a>.</div>
  17 <div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div>
  18
  19 <p>
  20   This is a quick and dirty document to get you on your way to writing
  21   custom URI filters for your own URL filtering needs.  Why would you
  22   want to write a URI filter?  If you need URIs your users put into
  23   HTML to magically change into a different URI, this is
  24   exactly what you need!
  25 </p>
  26
  27 <h2>Creating the class</h2>
  28
  29 <p>
  30   Any URI filter you make will be a subclass of <code>HTMLPurifier_URIFilter</code>.
  31   The scaffolding is thus:
  32 </p>
  33
  34 <pre>class HTMLPurifier_URIFilter_<strong>NameOfFilter</strong> extends HTMLPurifier_URIFilter
  35 {
  36     var $name = '<strong>NameOfFilter</strong>';
  37     function prepare($config) {}
  38     function filter(&$uri, $config, &$context) {}
  39 }</pre>
  40
  41 <p>
  42   Fill in the variable <code>$name</code> with the name of your filter, and
  43   take a look at the two methods. <code>prepare()</code> is an initialization
  44   method that is called only once, before any filtering has been done of the
  45   HTML. Use it to perform any costly setup work that only needs to be done
  46   once. <code>filter()</code> is the guts and innards of our filter:
  47   it takes the URI and does whatever needs to be done to it.
  48 </p>
  49
  50 <p>
  51   If you've worked with HTML Purifier, you'll recognize the <code>$config</code>
  52   and <code>$context</code> parameters.  On the other hand, <code>$uri</code>
  53   is something unique to this section of the application: it's a
  54   <code>HTMLPurifier_URI</code> object. The interface is thus:
  55 </p>
  56
  57 <pre>class HTMLPurifier_URI
  58 {
  59     var $scheme, $userinfo, $host, $port, $path, $query, $fragment;
  60     function HTMLPurifier_URI($scheme, $userinfo, $host, $port, $path, $query, $fragment);
  61     function toString();
  62     function copy();
  63     function getSchemeObj($config, &$context);
  64     function validate($config, &$context);
  65 }</pre>
  66
  67 <p>
  68   The first three methods are fairly self-explanatory: you have a constructor,
  69   a serializer, and a cloner.  Generally, you won't be using them when
  70   you are manipulating the URI objects themselves.
  71   <code>getSchemeObj()</code> is a special purpose method that returns
  72   a <code>HTMLPurifier_URIScheme</code> object corresponding to the specific
  73   URI at hand. <code>validate()</code> performs general-purpose validation
  74   on the internal components of a URI. Once again, you don't need to
  75   worry about these: they've already been handled for you.
  76 </p>
  77
  78 <h2>URI format</h2>
  79
  80 <p>
  81   As a URIFilter, we're interested in the member variables of the URI object.
  82 </p>
  83
  84 <table class="quick"><tbody>
  85   <tr><th>Scheme</th>   <td>The protocol for identifying (and possibly locating) a resource (http, ftp, https)</td></tr>
  86   <tr><th>Userinfo</th> <td>User information such as a username (bob)</td></tr>
  87   <tr><th>Host</th>     <td>Domain name or IP address of the server (example.com, 127.0.0.1)</td></tr>
  88   <tr><th>Port</th>     <td>Network port number for the server (80, 12345)</td></tr>
  89   <tr><th>Path</th>     <td>Data that identifies the resource, possibly hierarchical (/path/to, ed@example.com)</td></tr>
  90   <tr><th>Query</th>    <td>String of information to be interpreted by the resource (?q=search-term)</td></tr>
  91   <tr><th>Fragment</th> <td>Additional information for the resource after retrieval (#bookmark)</td></tr>
  92 </tbody></table>
  93
  94 <p>
  95   Because the URI is presented to us in this form, and not
  96   <code>http://bob@example.com:8080/foo.php?q=string#hash</code>, it saves us
  97   a lot of trouble in having to parse the URI every time we want to filter
  98   it. For the record, the above URI has the following components:
  99 </p>
 100
 101 <table class="quick"><tbody>
 102   <tr><th>Scheme</th>   <td>http</td></tr>
 103   <tr><th>Userinfo</th> <td>bob</td></tr>
 104   <tr><th>Host</th>     <td>example.com</td></tr>
 105   <tr><th>Port</th>     <td>8080</td></tr>
 106   <tr><th>Path</th>     <td>/foo.php</td></tr>
 107   <tr><th>Query</th>    <td>q=string</td></tr>
 108   <tr><th>Fragment</th> <td>hash</td></tr>
 109 </tbody></table>
 110
 111 <p>
 112   Note that there is no question mark or octothorpe in the query or
 113   fragment: these get removed during parsing.
 114 </p>
 115
 116 <p>
 117   With this information, you can get straight to implementing your
 118   <code>filter()</code> method. But one more thing...
 119 </p>
 120
 121 <h2>Return value: Boolean, not URI</h2>
 122
 123 <p>
 124   You may have noticed that the URI is being passed in by reference.
 125   This means that whatever changes you make to it, those changes will
 126   be reflected in the URI object the callee had.  <strong>Do not
 127   return the URI object: it is unnecessary and will cause bugs.</strong>
 128   Instead, return a boolean value, true if the filtering was successful,
 129   or false if the URI is beyond repair and needs to be axed.
 130 </p>
 131
 132 <p>
 133   Let's suppose I wanted to write a filter that de-internationalized domain
 134   names by converting them to <a href="http://en.wikipedia.org/wiki/Punycode">Punycode</a>.
 135   Assuming that <code>punycode_encode($input)</code> converts <code>$input</code> to
 136   Punycode and returns <code>false</code> on failure:
 137 </p>
 138
 139 <pre>class HTMLPurifier_URIFilter_ConvertIDNToPunycode extends HTMLPurifier_URIFilter
 140 {
 141     var $name = 'ConvertIDNToPunycode';
 142     function filter(&$uri, $config, &$context) {
 143         if (is_null($uri->host)) return true;
 144         if ($uri->host == utf8_decode($uri->host)) {
 145             // is ASCII, abort
 146             return true;
 147         }
 148         $host = punycode_encode($uri->host);
 149         if ($host === false) return false;
 150         $uri->host = $host;
 151         return true;
 152     }
 153 }</pre>
 154
 155 <p>
 156   Notice I did not <code>return $uri;</code>.
 157 </p>
 158
 159 <h2>Activating your filter</h2>
 160
 161 <p>
 162   Having a filter is all well and good, but you need to tell HTML Purifier
 163   to use it. Fortunately, this part's simple:
 164 </p>
 165
 166 <pre>$uri =& $config->getDefinition('URI');
 167 $uri->addFilter(new HTMLPurifier_URIFilter_<strong>NameOfFilter</strong>());</pre>
 168
 169 <p>
 170   If you want to be really fancy, you can define a configuration directive
 171   for your filter and have HTML Purifier automatically manage whether or
 172   not your filter gets loaded or not (this is how internal filters manage
 173   things):
 174 </p>
 175
 176 <pre>HTMLPurifier_ConfigSchema::define(
 177     'URI', '<strong>NameOfFilter</strong>', false, 'bool',
 178     '<strong>What your filter does.</strong>'
 179 );
 180 $uri =& $config->getDefinition('URI', true);
 181 $uri->registerFilter(new HTMLPurifier_URIFilter_<strong>NameOfFilter</strong>());
 182 </pre>
 183
 184 <p>
 185   Now, your filter will only be called when %URI.<strong>NameOfFilter</strong>
 186   is set to true.
 187 </p>
 188
 189 <h2>Examples</h2>
 190
 191 <p>
 192   Check the
 193   <a href="http://htmlpurifier.org/svnroot/htmlpurifier/trunk/library/HTMLPurifier/URIFilter/">URIFilter</a>
 194   directory for more implementation examples, and see <a href="http://htmlpurifier.org/svnroot/htmlpurifier/trunk/docs/proposal-new-directives.txt">the
 195   new directives proposal document</a> for ideas on what could be implemented
 196   as a filter.
 197 </p>
 198
 199 <div id="version">$Id$</div>
 200
 201 </body></html>