As @fbernodi said earlier, there is a problem with saveXML of DOMNode when you have a number of defined namespaces. The simple solution for this:
<code>
// $node is some node of some other document
$temp_document = new DOMDocument('1.0', 'utf-8');
$temp_document->appendChild($temp_document->importNode($node, true));
echo $temp_document->saveXML();
</code>