To avoid script tags from being output as <script />, you can use the DOMDocumentFragment class:
<?php
$doc = new DOMDocument();
$doc -> loadXML($xmlstring);
$fragment = $doc->createDocumentFragment();
/* Append the script element to the fragment using raw XML strings (will be preserved in their raw form) and if succesful proceed to insert it in the DOM tree */
if($fragment->appendXML("<script type='text/javascript' src='$source'></script>") {
$xpath = new DOMXpath($doc);
$resultlist = $xpath->query("//*[local-name() = 'html']/*[local-name() = 'head']"); /* namespace-safe method to find all head elements which are childs of the html element, should only return 1 match */
foreach($resultlist as $headnode) // insert the script tag
$headnode->appendChild($fragment);
}
$doc->saveXML(); /* and our script tags will still be <script></script> */
?>