public function sendEcho($cb, $data = 'phpdaemon')
    {
        ++$this->seq;
        if (mb_orig_strlen($data) % 2 !== 0) {
            $data .= "\x00";
        }

        $packet = pack(
                'ccnnn',
                8, // type (c)
                0, // code (c)
                0, // checksum (n)
                Daemon::$process->getPid(), // pid (n)
                $this->seq // seq (n)
            ) . $data;

        $packet = substr_replace($packet, self::checksum($packet), 2, 2);
        $this->write($packet);
        $this->onResponse->push([$cb, microtime(true)]);
    }