<?php

/*
<data>
<menu xml:id="bar0" tpl="" name="Impressum" link="" target="" secure="">
        <site xml:id="foo0" tpl="index" name="Datenschutz" link="" target="" secure=""/>
        <site xml:id="foo1" tpl="index" name="Datenschutz" link="" target="" secure=""/>
        <site xml:id="foo2" tpl="index" name="Datenschutz" link="" target="" secure=""/>
    <menu xml:id="bar1" tpl="" name="Impressum" link="" target="" secure="">
            <site xml:id="foo10" tpl="index" name="Datenschutz" link="" target="" secure=""/>
            <site xml:id="foo11" tpl="index" name="Datenschutz" link="" target="" secure=""/>
            <site xml:id="foo12" tpl="index" name="Datenschutz" link="" target="" secure=""/>
    </menu>
</menu>
</data>
*/
$doc = new DomDocument;
$doc->validateOnParse = true;
$doc->load('php.xml');

//header('Content-Type: text/xml');
//echo $doc->saveXML();

var_dump($doc->getElementById('bar1')->tagName); // menu

$sites = $doc->getElementsByTagName('site');

foreach ($sites as $site)
{

    if($site->getAttribute('xml:id') != 'foo11')
        continue;

    echo $site->parentNode->getAttribute('xml:id'); //bar1
}