Hello,

I'm trying to extend two native PHP5 classes (DOMDocument and DOMNode) to implement 2 methods (selectNodes and selectSingleNode) in order to make XPath queries easier. I thought it would be rather straighforward, but I'm stuck in a problem which I think is an OOP beginner's issue.

class nDOMDocument extends DOMDocument {
    public function selectNodes($xpath){
    $oxpath = new DOMXPath($this);
    return $oxpath->query($xpath);
  }
  public function selectSingleNode($xpath){
    return $this->selectNodes($xpath)->item(0);
  }
}

Then I tried to do extend DOMNode to implement the same methods so I can perform an XPath query directly on a node:

class nDOMNode extends DOMNode {
    public function selectNodes($xpath){
    $oxpath = new DOMXPath($this->ownerDocument,$this);
    return $oxpath->query($xpath);
  }
  public function selectSingleNode($xpath){
    return $this->selectNodes($xpath)->item(0);
  }
}

Now if I execute the following code (on an arbitrary XMLDocument):

$xmlDoc = new nDOMDocument;
$xmlDoc->loadXML(...some XML...);
$node1 = $xmlDoc->selectSingleNode("//item[@id=2]");
$node2 = $node1->selectSingleNode("firstname");

The third line works and returns a DOMNode object $node1. However, the fourth line doesn't work because the selectSingleNode method belongs to the nDOMNode class, not DOMNode. So my question: is there a way at some point to "transform" the returned DOMNode object into a nDOMNode object? I feel I'm missing some essential point here and I'd greatly appreciate your help.

(Sorry, this is a restatement of my question http://stackoverflow.com/questions/2573820/)