GetElementById Pitfalls

Intro

Contrary to popular belief, getElementById() (W3 spec, PHP manual entry) does not return the element with the attribute named <em>id</em> by default. According to the DOM specs, getElementById does return the element with an attribute defined as attribute type ID. In HTML (and therefore all HTML DOM implementations in browsers, SVG is another example), this happens to be the attribute named id. But if you want to use getElementById with an arbitrary XML document, you have to define that first. But there's also an easy solution with the recent addition of xml__id support in libxml2, at least if you're in control of the XML documents.

Following are different approaches to make getElementById work with your XML documents (the examples are for PHP 5, but the techniques should work with other languages as well)

DTD

You can define your ID attributes with a DTD

$xml ='<?xml version="1.0"?>
{panel}
  <!DOCTYPE foo [
  <!ATTLIST bar
    id ID #IMPLIED
  >
  ]>
  <foo>
    <bar id="myId"/>
  </foo>';
{panel}
$dom = new DomDocument();
$dom->loadXML($xml);
print $dom->getElementById('myId')->nodeName;

You have to define the ID attribute for every element. And your ID attribute doesn't have to be called id, it can be every valid attribute name. Of course, the DTD definition can also be external.

Here's how it's defined in the XHTML 1.0 DTD -> http://www.w3.org/TR/xhtml1/dtds.html#dtdentry_xhtml1-strict.dtd_coreattrs

XML Schema

Instead of a DTD, you can also use a XML Schema document. I'm no Schema expert, therefore the following is just a snippet of how it should basically look like. Any additions to it are welcome

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

{panel}
    <xs:attribute name="id" type="xs:ID"/>
{panel}

</xs:schema>

Relax NG

In Relax NG, you can also define ID attributes with a data-type expression:

{panel}
 <attribute name="id">
      <data type="ID"/>
   </attribute>
{panel}

Also untested.

xml:id

As the above scenarios all have their drawbacks (it relies on external definitions of what's an ID and also needs a validating parser), the W3C came up with another approach, the xml:id attribute. The specs are quite short and worth a read for more information about the whole problem.

As of version 2.6.9, libxml2 (and therefore PHP) does support the xml:id specs:

$xml ='<?xml version="1.0"?>  
{panel}
  <foo>
    <bar xml:id="myId"/>
  </foo>';
{panel}
$dom = new DomDocument();
$dom->loadXML($xml);
print $dom->getElementById('myId')->nodeName;

The ID does have to be a valid NCName, which for example means, that the first letter can't be a number.

Addition from discussion on php-xml-dev:

> DOMDocument::getElementByID() doesn't work with xml:id 
> if the DOMDocument is being created in code.
>
> It only works when the document is either loaded from a 
> string or from a file.
>
> As per the discussion, this is an issue with libxml2. 
> As of libxml2.6.17, it still doesn't work.

And will never work that way. IDs would need to be set 
using the DOMElement setIdAttribute methods, but these 
aren't currently implemented as there are some libxml 
issues surrounding this.

XPath

If you can't change the XML document and use xml:id (or adding DTD definitions) or you don't have a validating parser, you can also use XPath for getting elements with an attribute named id (this would also work with @xml:id instead of @id, if you don't have libxml2 2.6.9 installed).

$xml ='<?xml version="1.0"?>  
{panel}
  <foo>
    <bar id="myId"/>
  </foo>';
{panel}
$dom = new DomDocument();
$dom->loadXML($xml);
$xp = new DomXPath($dom);
$res = $xp->query("//*[@id = 'myId']");
print $res->item(0)->nodeName;

This does actually return the element with an attribute named id in contrast to the DTD approach above, which returns the element with the attribute defined as ID. You should be aware of that difference.

Benchmarks

See my slides about "XML on Speed" to see some benchmarks with XPath vs. getElementById() : http://php5.bitflux.org/xmlonspeed/slide_21.php

GetElementById Pitfalls

Intro

DTD

XML Schema

Relax NG

xml:id

XPath

Benchmarks

More Links

Labels

Comments (3)

Anonymous says:

Anonymous says:

Anonymous says:

Add Labels
Enter labels to add to this page: Looking for a label? Just start typing.