Using;
libxml Version
2.4.21

HTML Support
enabled

XPath Support
enabled

XPointer Support
enabled

The following results in "call to undefined function";

<?php
$dom=domxml_new_doc('1.0');
$root=$dom->create_element_ns('http://www.php.net','domxml','php');
$root=$dom->append_child($root);
?>

Using create_element though causes no problems;

<?php
$dom=domxml_new_doc('1.0');
$root=$dom->create_element('domxml');
$root=$dom->append_child($root);
?>

[Looked hard but found no other bug listed with reference to create_element_ns in the DOM extension]