XMLSecurityDSig::locateSignature PHP Method

    public function locateSignature($objDoc, $pos = 0)
    {
        if ($objDoc instanceof DOMDocument) {
            $doc = $objDoc;
        } else {
            $doc = $objDoc->ownerDocument;
        }
        if ($doc) {
            $xpath = new DOMXPath($doc);
            $xpath->registerNamespace('secdsig', XMLSecurityDSig::XMLDSIGNS);
            $query = ".//secdsig:Signature";
            $nodeset = $xpath->query($query, $objDoc);
            $this->sigNode = $nodeset->item($pos);
            return $this->sigNode;
        }
        return null;
    }

 public function is_valid()
 {
     $objXMLSecDSig = new XMLSecurityDSig();
     $objDSig = $objXMLSecDSig->locateSignature($this->document);
     if (!$objDSig) {
         throw new Exception("Cannot locate Signature Node");
     }
     $objXMLSecDSig->canonicalizeSignedInfo();
     $objXMLSecDSig->idKeys = array('ID');
     $retVal = $objXMLSecDSig->validateReference();
     if (!$retVal) {
         throw new Exception("Reference Validation Failed");
     }
     $objKey = $objXMLSecDSig->locateKey();
     if (!$objKey) {
         throw new Exception("We have no idea about the key");
     }
     $key = null;
     $singleAssertion = $this->validateNumAssertions();
     if (!$singleAssertion) {
         throw new Exception("Only one SAMLAssertion allowed");
     }
     $validTimestamps = $this->validateTimestamps();
     if (!$validTimestamps) {
         throw new Exception("SAMLAssertion conditions not met");
     }
     $objKeyInfo = XMLSecEnc::staticLocateKeyInfo($objKey, $objDSig);
     $objKey->loadKey($this->settings->x509certificate, false, true);
     $result = $objXMLSecDSig->verify($objKey);
     return $result;
 }