note 86056 modified in domdocument.getelementbyid by danbrown

You don't want to use "xml:id" ?
Here is the relaxNG trick (with a generic schema):
(tested with libxml 2.6.26)

<?php
$doc = new DOMDocument();
$doc->load(...);

$rng = '
<grammar xmlns="http://relaxng.org/ns/structure/1.0" datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
	<start>
		<element>
			<anyName/>
			<ref name="anythingID"/>
		</element>
	</start>
	<define name="anythingID">
		<zeroOrMore>
			<choice>
				<element>
					<anyName/>
					<ref name="anythingID"/>
				</element>
				<attribute name="id">
					<data type="ID"/>
				</attribute>
				<zeroOrMore>
					<attribute><anyName/></attribute>
				</zeroOrMore>
				<text/>
			</choice>
		</zeroOrMore>
	</define>
</grammar>
';

$doc->relaxNGValidateSource($rng);
var_dump($doc->getElementById('id1'));
?>


Note that ID values must be valid ones :
  - integers do no work! 
  - @see http://www.w3.org/TR/REC-xml/#id
  - => (Letter | '_' | ':') ( Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender  )*

--was--
You don't want to use "xml:id" ?
Here is the relaxNG trick (with a generic schema):
(tested with libxml 2.6.26)

$doc = new DOMDocument();
$doc->load(...);

$rng = '
<grammar xmlns="http://relaxng.org/ns/structure/1.0" datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
	<start>
		<element>
			<anyName/>
			<ref name="anythingID"/>
		</element>
	</start>
	<define name="anythingID">
		<zeroOrMore>
			<choice>
				<element>
					<anyName/>
					<ref name="anythingID"/>
				</element>
				<attribute name="id">
					<data type="ID"/>
				</attribute>
				<zeroOrMore>
					<attribute><anyName/></attribute>
				</zeroOrMore>
				<text/>
			</choice>
		</zeroOrMore>
	</define>
</grammar>
';

$doc->relaxNGValidateSource($rng);
var_dump($doc->getElementById('id1'));

Note that ID values must be valid ones :
  - integers do no work! 
  - @see http://www.w3.org/TR/REC-xml/#id
  - => (Letter | '_' | ':') ( Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender  )*

http://php.net/manual/en/domdocument.getelementbyid.php