Description:
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DOM extension for PHP supports XML entity references by implementing DOMEntityReference class. However due to incorrect entity name validation this class only allows working with named entity references, not character references.

libxml2, which is used as backend implementation for DOM XML operations have 2 functions for creating entity references:
xmlNewReference() - for entity references, it is used by DOM extension
xmlNewCharRef() - for character references, it is not used by DOM extension and hence causes extension to lack support for this kind of entities.

Moreover, implementation of DOMEntityReference::__construct() in ext/dom/entityreference.c uses libxml2 function xmlValidateName() for validating entity name which checks for Name (see http://www.w3.org/TR/REC-xml/#NT-EntityRef). Of course this check is failed on character references ( see http://www.w3.org/TR/REC-xml/#NT-CharRef) and hence causes exception or warning error to be thrown.

Correct implementation should check for "#" character at a first position of given entity name and call xmlNewReference() or xmlNewCharRef() depending on test result.

PHP 5.2.x is also affected by this problem.

Test script:
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<?php
$xml = new DOMDocument('1.0','utf-8');
$node = $xml->createElement('test');
$xml->appendChild($node);
$named = $xml->createEntityReference('entity');     // Create named entity, works
$node->appendChild($named);
$char = $xml->createEntityReference('#xAA');        // Create character reference, doesn't work
$node->appendChild($char);
echo $xml->saveXML();

Expected result:
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<?xml version="1.0" encoding="utf-8"?>
<test>&entity;&#xAA;</test>

Actual result:
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Fatal error: Uncaught exception 'DOMException' with message 'Invalid Character Error' in test.php:7
Stack trace:
#0 test.php(7): DOMDocument->createEntityReference('#xAA')
#1 {main}
  thrown in test.php on line 7