There are two ways I know of:
1 - there's a simple api for XML which is native to PHP4 - documented at http://www.php.net/manual/en/ref.xml.php
- It works well and it's stable, but there's no validation and it's not a DOM parser - it has no concept of the nature of the document, it just stores the nodes, attributes and values in PHP variables. My experience was initially very positive, but I was more interested in XSLT ...
2 - PHP has XSLT function - documented at http://www.php.net/manual/en/ref.xslt.php
- for this you need to install an extension called Sablotron - http://www.gingerall.com/charlie/ga/xml/x_sabphp.xml
- and it truly rocks
Really simple - takes about 4 lines of PHP, which is particularly great if you want environment independence for your data - and it's a true DOM parser with full XPath support. This is my recomendation