I had some difficulty stripping all default NS attributes for an ns-uri in one shot, the following will work though...first strip the documentElement namespace, then getElementsByTagNameNS() -- the documentation should reflect that the 2nd argument is actually the name of the tag, not the local namespace prefix as I first expected:

<?php

function strip_default_ns( $xml = null, $ns_uri = 'http://example.com/XML-Foo' ) {
    $ns_local = '';
    $ns_tag = '*';
    
    if ( empty($xml) ) return false;
    
    //remove document namespace
    $dom = new DOMDocument();
    $dom->loadXML($xml);
    $dom->documentElement->removeAttributeNS($ns_uri, $ns_local);
    
    //strip element namespaces
    foreach ( $dom->getElementsByTagNameNS($ns_uri, $ns_tag) as $elem ) {
        $elem->removeAttributeNS($ns_uri, $ns_local);
    }

    return $dom->saveXML();
}

$stripped_xml = strip_default_ns($the_xml);

?>

$stripped_xml can now take advantage of running XPath queries on it for the NULL namespace.