Xpath fails if an element has a a xmlns attribute [duplicate]

Answer 1

The namespace declaration means that the names are in a namespace, but your XPath is looking for names in no namespace. The way you search for namespaced elements depends on whether you are using XPath 1.0 or 2.0, and to the extent that you use namespace prefixes in your XPath to represent namespaced names, the way that you declare the prefixes depends on the API to the XPath processor you are using.

Answer 2

You need to register the namespace:

$lol = new SimpleXMLElement($string);
$lol->registerXPathNamespace('cada', 'http://www.collada.org/2005/11/COLLADASchema');
print_r($lol->xpath("child::cada:library_materials//*[@id='Material2']"));