$doc->findnodes('//myns:root/myns:stuff/myns:items/myns:book');


##</code><code>##


<root xmlns="http://goobar/xml" xmlns:myns="http://goobar/xml">
  <stuff>
    <items>
       <book>XML Namespaces</book>
    </items>
  </stuff>
</root>