Please try using this CVS snapshot:

  http://snaps.php.net/php4-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php4-win32-latest.zip

I got the same problem also. I'm using ZendEngine2 (PHP4.3.0_zend2_alpha3). Following are my two test cases.

1. $elm = new DomElement("element"); 
It returns an DomElement object with NO property. I traced the source code, found that the internal constructor function domxml_doc_create_element() was not invoked. 

2. $elm = $doc->create_element("element"); 
The method returns NULL. Furthermore, the internal function domxml_doc_create_element() applied on the wrong object $doc instead of $elm ($doc's type changed to XML_ELEMENT_NODE, previously it was XML_DOCUMENT_NODE ). Dump of $doc is like this:
domdocument Object
(
    [name] => #document
    [url] => 
    [version] => 1.0
    [standalone] => -1
    [type] => 1
    [compression] => -1
    [charset] => 1
    [0] => 2
    [1] => 136550520
    [tagname] => element
)

Above bugs exist in all Dom* classes and create_* methods.

After I compiled php4 with zend1.3.0, above test cases passed, everything became OK. So I believe it is some kind of bug within ZendEnging2.

No feedback was provided for this bug for over 2 weeks, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

This problem exists under my setup of PHP 5.0.5, its associated php_domxml.dll module for the same version of PHP, and Windows XP Professional served under Apache 2.0.54. create_element() as well as create_element_ns() return NULL, without so much as a notice. In fact, I have never been able to get it to return anything but NULL.

Another example, even simpler, that reproduces the error:
<?php
	$doc = domxml_new_doc("1.0");
	$root = $doc->create_element("notifications");
	
	echo "<pre>";
	var_dump($root);
	echo "</pre>";
?>
The output is "NULL."

I have the same problem on Apache 2.0.59 with PHP 5.2.1 on Widnows XP 32 bit.