public static function getIssuer($request)
    {
        if ($request instanceof DOMDocument) {
            $dom = $request;
        } else {
            $dom = new DOMDocument();
            $dom = OneLogin_Saml2_Utils::loadXML($dom, $request);
        }

        $issuer = null;
        $issuerNodes = OneLogin_Saml2_Utils::query($dom, '/samlp:LogoutRequest/saml:Issuer');
        if ($issuerNodes->length == 1) {
            $issuer = $issuerNodes->item(0)->textContent;
        }
        return $issuer;
    }