$policy_utf = utf8_encode($policy);//ensure utf-8
$policy_base64 = base64_encode($policy_utf);//encoded by base64

$signature = googleSignString('docker-71ea7c875c5e.p12', $policy_base64);

/*
 * @params p12FilePath Service account p12 file path
 * @params string Plain text for encoding
 */
function googleSignString($p12FilePath, $string){
        $certs = [];
        if (!openssl_pkcs12_read(file_get_contents($p12FilePath), $certs, 'notasecret'))
        {
            echo "Unable to parse the p12 file. OpenSSL error: " . openssl_error_string(); exit();
        }
        $RSAPrivateKey = openssl_pkey_get_private($certs["pkey"]);
        $signed = '';
        if(!openssl_sign( $string, $signed, $RSAPrivateKey, 'sha256' ))
        {
            error_log( 'openssl_sign failed!' );
            $signed = 'failed';
        }
        else
        {
            $signed = base64_encode($signed);
        }
        return $signed;
    }