public function terminate()
    {
        if (function_exists('posix_kill')) {
            posix_kill($this->pid, 15);
        } elseif (PHP_OS === 'WINNT') {
            exec('taskkill /PID ' . escapeshellarg($this->pid));
        } else {
            exec('kill -15 ' . escapeshellarg($this->pid));
        }
    }