how i iterate by dom elements php?

i have an xml record hallowed dom document,
i wish iterate by 'foo' tags, removing values each add-on next it. i know i values around

$element = $dom->getelementsbytagname('foo')->item(0);
foreach($element->childnodes $node){
    $data[$node->nodename] = $node->nodevalue;
}

however, i'm perplexing do, an xml like,

<stuff>
  <foo>
    <bar></bar>
      <value/>
    <pub></pub>
  </foo>
  <foo>
    <bar></bar>
    <pub></pub>
  </foo>
  <foo>
    <bar></bar>
    <pub></pub>
  </foo>
  </stuff>

iterate over each foo tag, specific bar pub, values there.
now, i iterate over foo i still opening specific child nodes name?