PHP and XML
PHP includes optional XML extensions that support the Expat parser. XML-related functions in PHP allow you to create a parser to process valid XML documents. If the Apache version you are using is later than 1.3.7, then you do not need other additional libraries. And all you need to do is configure PHP--with-xml.
The PHP manual has several excellent examples of creating PHP-based XML parsers. In addition, the New Riders book Web Application Development with PHP 4.0 also has a detailed explanation of the use of XML with PHP, and one of the authors of this number has written quite a few guides in this regard on Zend.com.
Like Java and COM support, PHP support for XML is still in its infancy, but this area is growing rapidly. If you already have experience with Expat or LibXML, you can skip the above and directly leverage your expertise.