I need to query an xml document and then display specific tag values, e.g. forename, surname, group(dept), job_title.

I'm using XMLReader as i may need to work with large XML files. I using DomXPath to filter the data, but i don't know how to retrieve the nodeName and value for each element. The code below only returns 'member' as the node name?

Any help would be appreciated.

<?php
    $reader = new XMLReader();
    $reader->open('include/staff.xml');

    while ($reader->read()){
        switch($reader->nodeType){
            case(XMLREADER::ELEMENT):
                if($reader->localName === 'staff'){
                    $node = $reader->expand();
                    $dom = new DomDocument();
                    $dom->formatOutput = true;
                    $n = $dom->importNode($node, true);
                    $dom->appendChild($n);
                    $xp = new DomXpath($dom);
                    $res = $xp->query("/staff/member[groups='HR']");
                }
        }
    }
    echo $res->item(0)->nodeName;
    echo $res->item(0)->nodeValue;
?>