XMLSecurityDSig::insertSignature PHP Method

    public function insertSignature($node, $beforeNode = null)
    {
        $document = $node->ownerDocument;
        $signatureElement = $document->importNode($this->sigNode, true);
        if ($beforeNode == null) {
            return $node->insertBefore($signatureElement);
        } else {
            return $node->insertBefore($signatureElement, $beforeNode);
        }
    }

 /**
  * Adds signature key and senders certificate to an element (Message or Assertion).
  *
  * @param string|DomDocument $xml  The element we should sign
  * @param string             $key  The private key
  * @param string             $cert The public
  */
 public static function addSign($xml, $key, $cert)
 {
     if ($xml instanceof DOMDocument) {
         $dom = $xml;
     } else {
         $dom = new DOMDocument();
         $dom = self::loadXML($dom, $xml);
         if (!$dom) {
             throw new Exception('Error parsing xml string');
         }
     }
     /* Load the private key. */
     $objKey = new XMLSecurityKey(XMLSecurityKey::RSA_SHA1, array('type' => 'private'));
     $objKey->loadKey($key, false);
     /* Get the EntityDescriptor node we should sign. */
     $rootNode = $dom->firstChild;
     /* Sign the metadata with our private key. */
     $objXMLSecDSig = new XMLSecurityDSig();
     $objXMLSecDSig->setCanonicalMethod(XMLSecurityDSig::EXC_C14N);
     $objXMLSecDSig->addReferenceList(array($rootNode), XMLSecurityDSig::SHA1, array('http://www.w3.org/2000/09/xmldsig#enveloped-signature', XMLSecurityDSig::EXC_C14N), array('id_name' => 'ID'));
     $objXMLSecDSig->sign($objKey);
     /* Add the certificate to the signature. */
     $objXMLSecDSig->add509Cert($cert, true);
     $insertBefore = $rootNode->firstChild;
     $messageTypes = array('samlp:AuthnRequest', 'samlp:Response', 'samlp:LogoutRequest', 'samlp:LogoutResponse');
     if (in_array($rootNode->tagName, $messageTypes)) {
         $issuerNodes = self::query($dom, '/' . $rootNode->tagName . '/saml:Issuer');
         if ($issuerNodes->length == 1) {
             $insertBefore = $issuerNodes->item(0)->nextSibling;
         }
     }
     /* Add the signature. */
     $objXMLSecDSig->insertSignature($rootNode, $insertBefore);
     /* Return the DOM tree as a string. */
     $signedxml = $dom->saveXML();
     return $signedxml;
 }