passing DOMDocument Object to another function

I am accessing information in my database. If the format is set to be html, I echo out the variables just fine. If the format is set to be xml, I create a DOMDocument. The problem is that in processing the result set, it calls several other functions. How do I pass the DOMDocument Object to these other functions? Do I need to pass the DOMElement Object to this other function? The "normal" ways don't seem to be working. I even just global'd the DOMDocument and DOMElement and that also didn't work. sample code:

         if($this->format=='xml')
         {
                  $dom=new DOMDocument('1.0', 'utf-8');
                  $menu_top=$dom->appendChild($dom->createElement('menu'));
                  print_r($dom);
          print_r($menu_top);
         }
         else
         {

              echo $row['menu_name'] . "<br>";

         }

         $this->renderChildrenMenu($row['id']);
    }

    if($this->format=='xml')
    {
        echo $dom->saveXML();
    }