public static function query($dom, $query, $context = null)
    {
        $xpath = new DOMXPath($dom);
        $xpath->registerNamespace('samlp', OneLogin_Saml2_Constants::NS_SAMLP);
        $xpath->registerNamespace('saml', OneLogin_Saml2_Constants::NS_SAML);
        $xpath->registerNamespace('ds', OneLogin_Saml2_Constants::NS_DS);
        $xpath->registerNamespace('xenc', OneLogin_Saml2_Constants::NS_XENC);
        $xpath->registerNamespace('xsi', OneLogin_Saml2_Constants::NS_XSI);
        $xpath->registerNamespace('xs', OneLogin_Saml2_Constants::NS_XS);
        $xpath->registerNamespace('md', OneLogin_Saml2_Constants::NS_MD);

        if (isset($context)) {
            $res = $xpath->query($query, $context);
        } else {
            $res = $xpath->query($query);
        }
        return $res;
    }