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.