Limitations of extending classes

• Scope!

   $dom = new books();
   $dom->load("books.xml");
   $e = $dom->documentElement;
   unset ($dom);
   $dom = $e->ownerDocument; // DomDocument, not book...
   

• Appending ..

   //Works somehow in this case, but is not the standard way
   class myelement extends DOMElement {};
   $domdoc = new DomDocument();
   $element = new myelement("fon");
   
   $domdoc->appendChild($element);