XML : PHP XPath how to wrap contents of p's in a span

I don't know if you can read JS Jquery but this is what I'd like to do server sided instead of client sided: $('p').wrapInner('<span class="contentsInP" />'); I'd like to take all existing paragraphs from a page and wrap their contents in a new span with a specific class.

Luckily all my documents are HTML5 in its XML flavour and are valid so that in PHP I can do this (simplified):

  $xml=new DOMDocument();  $xml->loadXML($html);  $xpath = new DOMXPath($xml);    // How to go on in here to wrap my p's?  $output=$xml->saveXML();    

How do I get PHP's DOMXPath to do my wrapping?