PHP XMLドキュメントの全ノードを巡る方法

<contact id="43956">
   <personal>
     <name>
        <first>J</first>
        <middle>J</middle>
        <last>J</last>
     </name>
     <title>Manager</title>
     <employer>National</employer>
     <dob>1971-12-22</dob>
   </personal>
</contact>

<?php
   function walk_tree ($node, $depth = 0) {
     for ($i = 0, $indent = ''; $i < $depth; $i++)
        $indent .= '   ';
     if ($node->type == XML_ELEMENT_NODE) {
        print ($indent . $node->tagname . "
");
        $kids = $node->children ();
        $nkids = count ($kids);
        if ($nkids > 0) {
           $depth++;
           for ($i = 0; $i < $nkids; $i++)
             walk_tree ($kids[$i], $depth);
           $depth--;
        }
     }
   }
   $doc = xmldocfile ('contact.xml');
   print ("<pre>
");
   walk_tree ($doc->root ());
   print ("</pre>
");
?>