public function kill() : bool
    {
        // kill server process:
        $serverPid = $this->getServerPid();
        if (!empty($serverPid)) {
            $this->killProcessById($serverPid);
        }

        // kill worker processes:
        $workerPids = [];
        foreach ($this->config['pool'] as $poolName => $poolConfig) {
            $pids = $this->getPidsByPath($poolConfig['worker_file']);
            if (empty($pids)) {
                continue;
            }
            $workerPids = array_merge($workerPids, $pids);
        }
        if (empty($workerPids)) {
            return true;
        }
        foreach ($workerPids as $pid) {
            $this->killProcessById($pid);
        }
        return true;
    }