Here's a very simple example on how to use PHP5 to transform a XML file using a XSL file.
<?php
   $xslDoc = new DOMDocument();
   $xslDoc->load("collection.xsl");
   $xmlDoc = new DOMDocument();
   $xmlDoc->load("collection.xml");
   $proc = new XSLTProcessor();
   $proc->importStylesheet($xslDoc);
   echo $proc->transformToXML($xmlDoc);
?>
For the sake of simplicity there's no error handling on this code. I hope this helps.