protected function bind()
    {
        if (function_exists('pcntl_signal')) {
            pcntl_signal(SIGTERM, [$this, 'shutdown']);
            pcntl_signal(SIGINT, [$this, 'shutdown']);
            pcntl_signal(SIGQUIT, [$this, 'shutdown']);
            pcntl_signal(SIGUSR2, [$this, 'pause']);
            pcntl_signal(SIGCONT, [$this, 'resume']);
        }
    }