docs/enduser-customize.html

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   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 customizing HTML Purifier's tag and attribute sets." />
   7 <link rel="stylesheet" type="text/css" href="style.css" />
   8
   9 <title>Customize - HTML Purifier</title>
  10
  11 </head><body>
  12
  13 <h1 class="subtitled">Customize!</h1>
  14 <div class="subtitle">HTML Purifier is a Swiss-Army Knife</div>
  15
  16 <div id="filing">Filed under End-User</div>
  17 <div id="index">Return to the <a href="index.html">index</a>.</div>
  18 <div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div>
  19
  20 <p>
  21   HTML Purifier has this quirk where if you try to allow certain elements or
  22   attributes, HTML Purifier will tell you that it's not supported, and that
  23   you should go to the forums to find out how to implement it. Well, this
  24   document is how to implement elements and attributes which HTML Purifier
  25   doesn't support out of the box.
  26 </p>
  27
  28 <h2>Is it necessary?</h2>
  29
  30 <p>
  31   Before we even write any code, it is paramount to consider whether or
  32   not the code we're writing is necessary or not. HTML Purifier, by default,
  33   contains a large set of elements and attributes: large enough so that
  34   <em>any</em> element or attribute in XHTML 1.0 or 1.1 (and its HTML variants)
  35   that can be safely used by the general public is implemented.
  36 </p>
  37
  38 <p>
  39   So what needs to be implemented? (Feel free to skip this section if
  40   you know what you want).
  41 </p>
  42
  43 <h3>XHTML 1.0</h3>
  44
  45 <p>
  46   All of the modules listed below are based off of the
  47   <a href="http://www.w3.org/TR/2001/REC-xhtml-modularization-20010410/abstract_modules.html#sec_5.2.">modularization of
  48   XHTML</a>, which, while technically for XHTML 1.1, is quite a useful
  49   resource.
  50 </p>
  51
  52 <ul>
  53   <li>Structure</li>
  54   <li>Frames</li>
  55   <li>Applets (deprecated)</li>
  56   <li>Forms</li>
  57   <li>Image maps</li>
  58   <li>Objects</li>
  59   <li>Frames</li>
  60   <li>Events</li>
  61   <li>Meta-information</li>
  62   <li>Style sheets</li>
  63   <li>Link (not hypertext)</li>
  64   <li>Base</li>
  65   <li>Name</li>
  66 </ul>
  67
  68 <p>
  69   If you don't recognize it, you probably don't need it. But the curious
  70   can look all of these modules up in the above-mentioned document.  Note
  71   that inline scripting comes packaged with HTML Purifier (more on this
  72   later).
  73 </p>
  74
  75 <h3>XHTML 1.1</h3>
  76
  77 <p>
  78   As of HTMLPurifier 2.1.0, we have implemented the
  79   <a href="http://www.w3.org/TR/2001/REC-ruby-20010531/">Ruby module</a>,
  80   which defines a set of tags
  81   for publishing short annotations for text, used mostly in Japanese
  82   and Chinese school texts, but applicable for positioning any text (not
  83   limited to translations) above or below other corresponding text.
  84 </p>
  85
  86 <h3>HTML 5</h3>
  87
  88 <p>
  89   <a href="http://www.whatwg.org/specs/web-apps/current-work/">HTML 5</a>
  90   is a fork of HTML 4.01 by WHATWG, who believed that XHTML 2.0 was headed
  91   in the wrong direction.  It too is a working draft, and may change
  92   drastically before publication, but it should be noted that the
  93   <code>canvas</code> tag has been implemented by many browser vendors.
  94 </p>
  95
  96 <h3>Proprietary</h3>
  97
  98 <p>
  99   There are a number of proprietary tags still in the wild. Many of them
 100   have been documented in <a href="ref-proprietary-tags.txt">ref-proprietary-tags.txt</a>,
 101   but there is currently no implementation for any of them.
 102 </p>
 103
 104 <h3>Extensions</h3>
 105
 106 <p>
 107   There are also a number of other XML languages out there that can
 108   be embedded in HTML documents: two of the most popular are MathML and
 109   SVG, and I frequently get requests to implement these.  But they are
 110   expansive, comprehensive specifications, and it would take far too long
 111   to implement them <em>correctly</em> (most systems I've seen go as far
 112   as whitelisting tags and no further; come on, what about nesting!)
 113 </p>
 114
 115 <p>
 116   Word of warning: HTML Purifier is currently <em>not</em> namespace
 117   aware.
 118 </p>
 119
 120 <h2>Giving back</h2>
 121
 122 <p>
 123   As you may imagine from the details above (don't be abashed if you didn't
 124   read it all: a glance over would have done), there's quite a bit that
 125   HTML Purifier doesn't implement.  Recent architectural changes have
 126   allowed HTML Purifier to implement elements and attributes that are not
 127   safe!  Don't worry, they won't be activated unless you set %HTML.Trusted
 128   to true, but they certainly help out users who need to put, say, forms
 129   on their page and don't want to go through the trouble of reading this
 130   and implementing it themself.
 131 </p>
 132
 133 <p>
 134   So any of the above that you implement for your own application could
 135   help out some other poor sap on the other side of the globe.  Help us
 136   out, and send back code so that it can be hammered into a module and
 137   released with the core.  Any code would be greatly appreciated!
 138 </p>
 139
 140 <h2>And now...</h2>
 141
 142 <p>
 143   Enough philosophical talk, time for some code:
 144 </p>
 145
 146 <pre>$config = HTMLPurifier_Config::createDefault();
 147 $config-&gt;set('HTML.DefinitionID', 'enduser-customize.html tutorial');
 148 $config-&gt;set('HTML.DefinitionRev', 1);
 149 $def = $config-&gt;getHTMLDefinition(true);</pre>
 150
 151 <p>
 152   Assuming that HTML Purifier has already been properly loaded (hint:
 153   include <code>HTMLPurifier.auto.php</code>), this code will set up
 154   the environment that you need to start customizing the HTML definition.
 155   What's going on?
 156 </p>
 157
 158 <ul>
 159   <li>
 160     The first three lines are regular configuration code:
 161     <ul>
 162       <li>
 163         %HTML.DefinitionID is set to a unique identifier for your
 164         custom HTML definition.  This prevents it from clobbering
 165         other custom definitions on the same installation.
 166       </li>
 167       <li>
 168         %HTML.DefinitionRev is a revision integer of your HTML
 169         definition.  Because HTML definitions are cached, you'll need
 170         to increment this whenever you make a change in order to flush
 171         the cache.
 172       </li>
 173     </ul>
 174   </li>
 175   <li>
 176     The fourth line retrieves a raw <code>HTMLPurifier_HTMLDefinition</code>
 177     object that we will be tweaking. If the parameter was removed, we
 178     would be retrieving a fully formed definition object, which is somewhat
 179     useless for customization purposes.
 180   </li>
 181 </ul>
 182
 183 <h3>Broken backwards-compatibility</h3>
 184
 185 <p>
 186   Those of you who have already been twiddling around with the raw
 187   HTML definition object, you'll be noticing that you're getting an error
 188   when you attempt to retrieve the raw definition object without specifying
 189   a DefinitionID.  It is vital to caching (see below) that you make a unique
 190   name for your customized definition, so make up something right now and
 191   things will operate again.
 192 </p>
 193
 194 <h2>Turn off caching</h2>
 195
 196 <p>
 197   To make development easier, we're going to temporarily turn off
 198   definition caching:
 199 </p>
 200
 201 <pre>$config = HTMLPurifier_Config::createDefault();
 202 $config-&gt;set('HTML.DefinitionID', 'enduser-customize.html tutorial');
 203 $config-&gt;set('HTML.DefinitionRev', 1);
 204 <strong>$config-&gt;set('Cache.DefinitionImpl', null); // TODO: remove this later!</strong>
 205 $def = $config-&gt;getHTMLDefinition(true);</pre>
 206
 207 <p>
 208   A few things should be mentioned about the caching mechanism before
 209   we move on.  For performance reasons, HTML Purifier caches generated
 210   <code>HTMLPurifier_Definition</code> objects in serialized files
 211   stored (by default) in <code>library/HTMLPurifier/DefinitionCache/Serializer</code>.
 212   A lot of processing is done in order to create these objects, so it
 213   makes little sense to repeat the same processing over and over again
 214   whenever HTML Purifier is called.
 215 </p>
 216
 217 <p>
 218   In order to identify a cache entry, HTML Purifier uses three variables:
 219   the library's version number, the value of %HTML.DefinitionRev and
 220   a serial of relevant configuration.  Whenever any of these changes,
 221   a new HTML definition is generated.  Notice that there is no way
 222   for the definition object to track changes to customizations: here, it
 223   is up to you to supply appropriate information to DefinitionID and
 224   DefinitionRev.
 225 </p>
 226
 227 <h2 id="addAttribute">Add an attribute</h2>
 228
 229 <p>
 230   For this example, we're going to implement the <code>target</code> attribute found
 231   on <code>a</code> elements.  To implement an attribute, we have to
 232   ask a few questions:
 233 </p>
 234
 235 <ol>
 236   <li>What element is it found on?</li>
 237   <li>What is its name?</li>
 238   <li>Is it required or optional?</li>
 239   <li>What are valid values for it?</li>
 240 </ol>
 241
 242 <p>
 243   The first three are easy: the element is <code>a</code>, the attribute
 244   is <code>target</code>, and it is not a required attribute. (If it
 245   was required, we'd need to append an asterisk to the attribute name,
 246   you'll see an example of this in the addElement() example).
 247 </p>
 248
 249 <p>
 250   The last question is a little trickier.
 251   Lets allow the special values: _blank, _self, _target and _top.
 252   The form of this is called an <strong>enumeration</strong>, a list of
 253   valid values, although only one can be used at a time.  To translate
 254   this into code form, we write:
 255 </p>
 256
 257 <pre>$config = HTMLPurifier_Config::createDefault();
 258 $config-&gt;set('HTML.DefinitionID', 'enduser-customize.html tutorial');
 259 $config-&gt;set('HTML.DefinitionRev', 1);
 260 $config-&gt;set('Cache.DefinitionImpl', null); // remove this later!
 261 $def = $config-&gt;getHTMLDefinition(true);
 262 <strong>$def->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');</strong></pre>
 263
 264 <p>
 265   The <code>Enum#_blank,_self,_target,_top</code> does all the magic.
 266   The string is split into two parts, separated by a hash mark (#):
 267 </p>
 268
 269 <ol>
 270   <li>The first part is the name of what we call an <code>AttrDef</code></li>
 271   <li>The second part is the parameter of the above-mentioned <code>AttrDef</code></li>
 272 </ol>
 273
 274 <p>
 275   If that sounds vague and generic, it's because it is!  HTML Purifier defines
 276   an assortment of different attribute types one can use, and each of these
 277   has their own specialized parameter format.  Here are some of the more useful
 278   ones:
 279 </p>
 280
 281 <table class="table">
 282   <thead>
 283     <tr>
 284       <th>Type</th>
 285       <th>Format</th>
 286       <th>Description</th>
 287     </tr>
 288   </thead>
 289   <tbody>
 290     <tr>
 291       <th>Enum</th>
 292       <td><em>[s:]</em>value1,value2,...</td>
 293       <td>
 294         Attribute with a number of valid values, one of which may be used. When
 295         s: is present, the enumeration is case sensitive.
 296       </td>
 297     </tr>
 298     <tr>
 299       <th>Bool</th>
 300       <td>attribute_name</td>
 301       <td>
 302         Boolean attribute, with only one valid value: the name
 303         of the attribute.
 304       </td>
 305     </tr>
 306     <tr>
 307       <th>CDATA</th>
 308       <td></td>
 309       <td>
 310         Attribute of arbitrary text. Can also be referred to as <strong>Text</strong>
 311         (the specification makes a semantic distinction between the two).
 312       </td>
 313     </tr>
 314     <tr>
 315       <th>ID</th>
 316       <td></td>
 317       <td>
 318         Attribute that specifies a unique ID
 319       </td>
 320     </tr>
 321     <tr>
 322       <th>Pixels</th>
 323       <td></td>
 324       <td>
 325         Attribute that specifies an integer pixel length
 326       </td>
 327     </tr>
 328     <tr>
 329       <th>Length</th>
 330       <td></td>
 331       <td>
 332         Attribute that specifies a pixel or percentage length
 333       </td>
 334     </tr>
 335     <tr>
 336       <th>NMTOKENS</th>
 337       <td></td>
 338       <td>
 339         Attribute that specifies a number of name tokens, example: the
 340         <code>class</code> attribute
 341       </td>
 342     </tr>
 343     <tr>
 344       <th>URI</th>
 345       <td></td>
 346       <td>
 347         Attribute that specifies a URI, example: the <code>href</code>
 348         attribute
 349       </td>
 350     </tr>
 351     <tr>
 352       <th>Number</th>
 353       <td></td>
 354       <td>
 355         Attribute that specifies an positive integer number
 356       </td>
 357     </tr>
 358   </tbody>
 359 </table>
 360
 361 <p>
 362   For a complete list, consult
 363   <a href="http://repo.or.cz/w/htmlpurifier.git?a=blob;hb=HEAD;f=library/HTMLPurifier/AttrTypes.php"><code>library/HTMLPurifier/AttrTypes.php</code></a>;
 364   more information on attributes that accept parameters can be found on their
 365   respective includes in
 366   <a href="http://repo.or.cz/w/htmlpurifier.git?a=tree;hb=HEAD;f=library/HTMLPurifier/AttrDef"><code>library/HTMLPurifier/AttrDef</code></a>.
 367 </p>
 368
 369 <p>
 370   Sometimes, the restrictive list in AttrTypes just doesn't cut it. Don't
 371   sweat: you can also use a fully instantiated object as the value. The
 372   equivalent, verbose form of the above example is:
 373 </p>
 374
 375 <pre>$config = HTMLPurifier_Config::createDefault();
 376 $config-&gt;set('HTML.DefinitionID', 'enduser-customize.html tutorial');
 377 $config-&gt;set('HTML.DefinitionRev', 1);
 378 $config-&gt;set('Cache.DefinitionImpl', null); // remove this later!
 379 $def = $config-&gt;getHTMLDefinition(true);
 380 <strong>$def-&gt;addAttribute('a', 'target', new HTMLPurifier_AttrDef_Enum(
 381   array('_blank','_self','_target','_top')
 382 ));</strong></pre>
 383
 384 <p>
 385   Trust me, you'll learn to love the shorthand.
 386 </p>
 387
 388 <h2>Add an element</h2>
 389
 390 <p>
 391   Adding attributes is really small-fry stuff, though, and it was possible
 392   to add them (albeit a bit more wordy) prior to 2.0. The real gem of
 393   the Advanced API is adding elements. There are five questions to
 394   ask when adding a new element:
 395 </p>
 396
 397 <ol>
 398   <li>What is the element's name?</li>
 399   <li>What content set does this element belong to?</li>
 400   <li>What are the allowed children of this element?</li>
 401   <li>What attributes does the element allow that are general?</li>
 402   <li>What attributes does the element allow that are specific to this element?</li>
 403 </ol>
 404
 405 <p>
 406   It's a mouthful, and you'll be slightly lost if your not familiar with
 407   the HTML specification, so let's explain them step by step.
 408 </p>
 409
 410 <h3>Content set</h3>
 411
 412 <p>
 413   The HTML specification defines two major content sets: Inline
 414   and Block.  Each of these
 415   content sets contain a list of elements: Inline contains things like
 416   <code>span</code> and <code>b</code> while Block contains things like
 417   <code>div</code> and <code>blockquote</code>.
 418 </p>
 419
 420 <p>
 421   These content sets amount to a macro mechanism for HTML definition. Most
 422   elements in HTML are organized into one of these two sets, and most
 423   elements in HTML allow elements from one of these sets.  If we had
 424   to write each element verbatim into each other element's allowed
 425   children, we would have ridiculously large lists; instead we use
 426   content sets to compactify the declaration.
 427 </p>
 428
 429 <p>
 430   Practically speaking, there are several useful values you can use here:
 431 </p>
 432
 433 <table class="table">
 434   <thead>
 435     <tr>
 436       <th>Content set</th>
 437       <th>Description</th>
 438     </tr>
 439   </thead>
 440   <tbody>
 441     <tr>
 442       <th>Inline</th>
 443       <td>Character level elements, text</td>
 444     </tr>
 445     <tr>
 446       <th>Block</th>
 447       <td>Block-like elements, like paragraphs and lists</td>
 448     </tr>
 449     <tr>
 450       <th><em>false</em></th>
 451       <td>
 452         Any element that doesn't fit into the mold, for example <code>li</code>
 453         or <code>tr</code>
 454       </td>
 455     </tr>
 456   </tbody>
 457 </table>
 458
 459 <p>
 460   By specifying a valid value here, all other elements that use that
 461   content set will also allow your element, without you having to do
 462   anything. If you specify <em>false</em>, you'll have to register
 463   your element manually.
 464 </p>
 465
 466 <h3>Allowed children</h3>
 467
 468 <p>
 469   Allowed children defines the elements that this element can contain.
 470   The allowed values may range from none to a complex regexp depending on
 471   your element.
 472 </p>
 473
 474 <p>
 475   If you've ever taken a look at the HTML DTD's before, you may have
 476   noticed declarations like this:
 477 </p>
 478
 479 <pre>&lt;!ELEMENT LI - O (%flow;)*             -- list item --&gt;</pre>
 480
 481 <p>
 482   The <code>(%flow;)*</code> indicates the allowed children of the
 483   <code>li</code> tag: <code>li</code> allows any number of flow
 484   elements as its children. (The <code>- O</code> allows the closing tag to be
 485   omitted, though in XML this is not allowed.) In HTML Purifier,
 486   we'd write it like <code>Flow</code> (here's where the content sets
 487   we were discussing earlier come into play). There are three shorthand
 488   content models you can specify:
 489 </p>
 490
 491 <table class="table">
 492   <thead>
 493     <tr>
 494       <th>Content model</th>
 495       <th>Description</th>
 496     </tr>
 497   </thead>
 498   <tbody>
 499     <tr>
 500       <th>Empty</th>
 501       <td>No children allowed, like <code>br</code> or <code>hr</code></td>
 502     </tr>
 503     <tr>
 504       <th>Inline</th>
 505       <td>Any number of inline elements and text, like <code>span</code></td>
 506     </tr>
 507     <tr>
 508       <th>Flow</th>
 509       <td>Any number of inline elements, block elements and text, like <code>div</code></td>
 510     </tr>
 511   </tbody>
 512 </table>
 513
 514 <p>
 515   This covers 90% of all the cases out there, but what about elements that
 516   break the mold like <code>ul</code>? This guy requires at least one
 517   child, and the only valid children for it are <code>li</code>. The
 518   content model is: <code>Required: li</code>. There are two parts: the
 519   first type determines what <code>ChildDef</code> will be used to validate
 520   content models. The most common values are:
 521 </p>
 522
 523 <table class="table">
 524   <thead>
 525     <tr>
 526       <th>Type</th>
 527       <th>Description</th>
 528     </tr>
 529   </thead>
 530   <tbody>
 531     <tr>
 532       <th>Required</th>
 533       <td>Children must be one or more of the valid elements</td>
 534     </tr>
 535     <tr>
 536       <th>Optional</th>
 537       <td>Children can be any number of the valid elements</td>
 538     </tr>
 539     <tr>
 540       <th>Custom</th>
 541       <td>Children must follow the DTD-style regex</td>
 542     </tr>
 543   </tbody>
 544 </table>
 545
 546 <p>
 547   You can also implement your own <code>ChildDef</code>: this was done
 548   for a few special cases in HTML Purifier such as <code>Chameleon</code>
 549   (for <code>ins</code> and <code>del</code>), <code>StrictBlockquote</code>
 550   and <code>Table</code>.
 551 </p>
 552
 553 <p>
 554   The second part specifies either valid elements or a regular expression.
 555   Valid elements are separated with horizontal bars (|), i.e.
 556   "<code>a | b | c</code>".  Use #PCDATA to represent plain text.
 557   Regular expressions are based off of DTD's style:
 558 </p>
 559
 560 <ul>
 561   <li>Parentheses () are used for grouping</li>
 562   <li>Commas (,) separate elements that should come one after another</li>
 563   <li>Horizontal bars (|) indicate one or the other elements should be used</li>
 564   <li>Plus signs (+) are used for a one or more match</li>
 565   <li>Asterisks (*) are used for a zero or more match</li>
 566   <li>Question marks (?) are used for a zero or one match</li>
 567 </ul>
 568
 569 <p>
 570   For example, "<code>a, b?, (c | d), e+, f*</code>" means "In this order,
 571   one <code>a</code> element, at most one <code>b</code> element,
 572   one <code>c</code> or <code>d</code> element (but not both), one or more
 573   <code>e</code> elements, and any number of <code>f</code> elements."
 574   Regex veterans should be able to jump right in, and those not so savvy
 575   can always copy-paste W3C's content model definitions into HTML Purifier
 576   and hope for the best.
 577 </p>
 578
 579 <p>
 580   A word of warning: while the regex format is extremely flexible on
 581   the developer's side, it is
 582   quite unforgiving on the user's side.  If the user input does not <em>exactly</em>
 583   match the specification, the entire contents of the element will
 584   be nuked.  This is why there is are specific content model types like
 585   Optional and Required: while they could be implemented as <code>Custom:
 586   (valid | elements)*</code>, the custom classes contain special recovery
 587   measures that make sure as much of the user's original content gets
 588   through. HTML Purifier's core, as a rule, does not use Custom.
 589 </p>
 590
 591 <p>
 592   One final note: you can also use Content Sets inside your valid elements
 593   lists or regular expressions. In fact, the three shorthand content models
 594   mentioned above are just that: abbreviations:
 595 </p>
 596
 597 <table class="table">
 598   <thead>
 599     <tr>
 600       <th>Content model</th>
 601       <th>Implementation</th>
 602     </tr>
 603   </thead>
 604   <tbody>
 605     <tr>
 606       <th>Inline</th>
 607       <td>Optional: Inline | #PCDATA</td>
 608     </tr>
 609     <tr>
 610       <th>Flow</th>
 611       <td>Optional: Flow | #PCDATA</td>
 612     </tr>
 613   </tbody>
 614 </table>
 615
 616 <p>
 617   When the definition is compiled, Inline will be replaced with a
 618   horizontal-bar separated list of inline elements. Also, notice that
 619   it does not contain text: you have to specify that yourself.
 620 </p>
 621
 622 <h3>Common attributes</h3>
 623
 624 <p>
 625   Congratulations: you have just gotten over the proverbial hump (Allowed
 626   children). Common attributes is much simpler, and boils down to
 627   one question: does your element have the <code>id</code>, <code>style</code>,
 628   <code>class</code>, <code>title</code> and <code>lang</code> attributes?
 629   If so, you'll want to specify the <code>Common</code> attribute collection,
 630   which contains these five attributes that are found on almost every
 631   HTML element in the specification.
 632 </p>
 633
 634 <p>
 635   There are a few more collections, but they're really edge cases:
 636 </p>
 637
 638 <table class="table">
 639   <thead>
 640     <tr>
 641       <th>Collection</th>
 642       <th>Attributes</th>
 643     </tr>
 644   </thead>
 645   <tbody>
 646     <tr>
 647       <th>I18N</th>
 648       <td><code>lang</code>, possibly <code>xml:lang</code></td>
 649     </tr>
 650     <tr>
 651       <th>Core</th>
 652       <td><code>style</code>, <code>class</code>, <code>id</code> and <code>title</code></td>
 653     </tr>
 654   </tbody>
 655 </table>
 656
 657 <p>
 658   Common is a combination of the above-mentioned collections.
 659 </p>
 660
 661 <p class="aside">
 662   Readers familiar with the modularization may have noticed that the Core
 663   attribute collection differs from that specified by the <a
 664   href="http://www.w3.org/TR/xhtml-modularization/abstract_modules.html#s_commonatts">abstract
 665   modules of the XHTML Modularization 1.1</a>. We believe this section
 666   to be in error, as <code>br</code> permits the use of the <code>style</code>
 667   attribute even though it uses the <code>Core</code> collection, and
 668   the DTD and XML Schemas supplied by W3C support our interpretation.
 669 </p>
 670
 671 <h3>Attributes</h3>
 672
 673 <p>
 674   If you didn't read the <a href="#addAttribute">earlier section on
 675   adding attributes</a>, read it now.  The last parameter is simply
 676   an array of attribute names to attribute implementations, in the exact
 677   same format as <code>addAttribute()</code>.
 678 </p>
 679
 680 <h3>Putting it all together</h3>
 681
 682 <p>
 683   We're going to implement <code>form</code>. Before we embark, lets
 684   grab a reference implementation from over at the
 685   <a href="http://www.w3.org/TR/html4/sgml/loosedtd.html">transitional DTD</a>:
 686 </p>
 687
 688 <pre>&lt;!ELEMENT FORM - - (%flow;)* -(FORM)   -- interactive form --&gt;
 689 &lt;!ATTLIST FORM
 690   %attrs;                              -- %coreattrs, %i18n, %events --
 691   action      %URI;          #REQUIRED -- server-side form handler --
 692   method      (GET|POST)     GET       -- HTTP method used to submit the form--
 693   enctype     %ContentType;  &quot;application/x-www-form-urlencoded&quot;
 694   accept      %ContentTypes; #IMPLIED  -- list of MIME types for file upload --
 695   name        CDATA          #IMPLIED  -- name of form for scripting --
 696   onsubmit    %Script;       #IMPLIED  -- the form was submitted --
 697   onreset     %Script;       #IMPLIED  -- the form was reset --
 698   target      %FrameTarget;  #IMPLIED  -- render in this frame --
 699   accept-charset %Charsets;  #IMPLIED  -- list of supported charsets --
 700   &gt;</pre>
 701
 702 <p>
 703   Juicy! With just this, we can answer four of our five questions:
 704 </p>
 705
 706 <ol>
 707   <li>What is the element's name? <strong>form</strong></li>
 708   <li>What content set does this element belong to? <strong>Block</strong>
 709     (this needs a little sleuthing, I find the easiest way is to search
 710     the DTD for <code>FORM</code> and determine which set it is in.)</li>
 711   <li>What are the allowed children of this element? <strong>One
 712     or more flow elements, but no nested <code>form</code>s</strong></li>
 713   <li>What attributes does the element allow that are general? <strong>Common</strong></li>
 714   <li>What attributes does the element allow that are specific to this element? <strong>A whole bunch, see ATTLIST;
 715     we're going to do the vital ones: <code>action</code>, <code>method</code> and <code>name</code></strong></li>
 716 </ol>
 717
 718 <p>
 719   Time for some code:
 720 </p>
 721
 722 <pre>$config = HTMLPurifier_Config::createDefault();
 723 $config-&gt;set('HTML.DefinitionID', 'enduser-customize.html tutorial');
 724 $config-&gt;set('HTML.DefinitionRev', 1);
 725 $config-&gt;set('Cache.DefinitionImpl', null); // remove this later!
 726 $def = $config-&gt;getHTMLDefinition(true);
 727 $def-&gt;addAttribute('a', 'target', new HTMLPurifier_AttrDef_Enum(
 728   array('_blank','_self','_target','_top')
 729 ));
 730 <strong>$form = $def-&gt;addElement(
 731   'form',   // name
 732   'Block',  // content set
 733   'Flow', // allowed children
 734   'Common', // attribute collection
 735   array( // attributes
 736     'action*' => 'URI',
 737     'method' => 'Enum#get|post',
 738     'name' => 'ID'
 739   )
 740 );
 741 $form-&gt;excludes = array('form' => true);</strong></pre>
 742
 743 <p>
 744   Each of the parameters corresponds to one of the questions we asked.
 745   Notice that we added an asterisk to the end of the <code>action</code>
 746   attribute to indicate that it is required. If someone specifies a
 747   <code>form</code> without that attribute, the tag will be axed.
 748   Also, the extra line at the end is a special extra declaration that
 749   prevents forms from being nested within each other.
 750 </p>
 751
 752 <p>
 753   And that's all there is to it! Implementing the rest of the form
 754   module is left as an exercise to the user; to see more examples
 755   check the <a href="http://repo.or.cz/w/htmlpurifier.git?a=tree;hb=HEAD;f=library/HTMLPurifier/HTMLModule"><code>library/HTMLPurifier/HTMLModule/</code></a> directory
 756   in your local HTML Purifier installation.
 757 </p>
 758
 759 <h2>And beyond...</h2>
 760
 761 <p>
 762   Perceptive users may have realized that, to a certain extent, we
 763   have simply re-implemented the facilities of XML Schema or the
 764   Document Type Definition.  What you are seeing here, however, is
 765   not just an XML Schema or Document Type Definition: it is a fully
 766   expressive method of specifying the definition of HTML that is
 767   a portable superset of the capabilities of the two above-mentioned schema
 768   languages.  What makes HTMLDefinition so powerful is the fact that
 769   if we don't have an implementation for a content model or an attribute
 770   definition, you can supply it yourself by writing a PHP class.
 771 </p>
 772
 773 <p>
 774   There are many facets of HTMLDefinition beyond the Advanced API I have
 775   walked you through today.  To find out more about these, you can
 776   check out these source files:
 777 </p>
 778
 779 <ul>
 780   <li><a href="http://repo.or.cz/w/htmlpurifier.git?a=blob;hb=HEAD;f=library/HTMLPurifier/HTMLModule.php"><code>library/HTMLPurifier/HTMLModule.php</code></a></li>
 781   <li><a href="http://repo.or.cz/w/htmlpurifier.git?a=blob;hb=HEAD;f=library/HTMLPurifier/ElementDef.php"><code>library/HTMLPurifier/ElementDef.php</code></a></li>
 782 </ul>
 783
 784 </body></html>
 785
 786 <!-- vim: et sw=4 sts=4
 787 -->