getElementById on dynamically created XML - php

Question

The example speaks for itself, I expect getElementById to return the second element but NULL is given. How come?
$dom = new DOMDocument();
$root = $dom->createElement("root");
$el = $dom->createElement("element");
$el->setAttribute("id", "1");
$root->appendChild($el);
$el = $dom->createElement("element");
$el->setAttribute("id", "2");
$root->appendChild($el);
$dom->appendChild($root);
// OK
echo $dom->saveXML();
// NOT OK
var_dump($dom->getElementById("2"));

Answer 1

With XML the getElementById method returns elements for which a certain attribute has been defined in the DTD as having type ID and not those named id. So you would need to make sure you have a DTD declaring the type ID for the elements named element and the attribute named id if you want to be able to use that method:
$dom = new DOMDocument();
$dom->loadXML("<!DOCTYPE root [<!ATTLIST element id ID #IMPLIED>]><root/>");
$root = $dom->documentElement;
It seems that in the PHP DOM API doing
$el = $dom->createElement("element");
$el->setAttribute("id", "2");
$el->setIdAttribute("id", TRUE);
serves as an alternative to having a DTD.

Answer 2

id is only an id attribute if defined by the DTD/XSD or API. The only predefined id attribute in XML is xml:id ({http://www.w3.org/XML/1998/namespace}id).
You can use Xpath to fetch an node by its attribute value. It does not need to by an id attribute for that:
$xml = <<<'XML'
<?xml version="1.0"?>
<root>
<element id="1"/>
<element id="2"/>
<element id="3"/>
</root>
XML;
$document = new DOMDocument();
$document->loadXml($xml);
$xpath = new DOMXpath($document);
var_dump(
$xpath->evaluate('//*[#id=2]')->item(0)->getAttribute('id')
);
Output:
string(1) "2"