Get here a good summary of the differences between PHP4 and PHP5 related to XML. Here you will find the ideas of various APIs and libraries available in PHP5 for XML.
Both ext/xslt and ext/domxml have been removed from PHP 4.
The goal for XML support in PHP 5 was not only to provide a solid base of tool sets for working with XML, but also to provide some unity amongst the tool sets themselves. The libxml2 and libxslt libraries, from the GNOME project, were chosen as the foundation for the XML support.