MultiClient TCP Server in PHP

gistfile1.php

<?php

$f = function ($input) {
    echo "input: " . $input;
};

try {
    // ポート4000、待機プロセス数2
    MultiClientTCPServer::instance(4000, 2)->run($f);
} catch (RuntimeException $e) {
    die('error: ' . $e->getMessage());
}

class MultiClientTCPServer
{
    private $process_stack = array();
    private $concurrency;
    private $port;

    private function __construct($port, $concurrency)
    {
        $this->port = $port;
        $this->concurrency = $concurrency;
    }

    public static function instance($port = 4000, $concurrency = 5)
    {
        return new self($port, $concurrency);
    }

    public function run($function_for_child)
    {
        $socket = socket_create_listen($this->port);

        while (true) {
            if (count($this->process_stack) >= $this->concurrency) {
                $stopped_pid = pcntl_waitpid(-1, $status, WUNTRACED);
                unset($this->process_stack[$stopped_pid]);
            }
            $pid = pcntl_fork();

            switch ($pid) {
            case -1:
                throw new RuntimeException('can not fork process');
                break;
            case 0:
                // child process
                $client = socket_accept($socket);
                while (true) {
                    $input = socket_read($client, 1024);
                    if (preg_match('/^exit;/', $input) === 1) break;
                    $result = $function_for_child($input);
                }
                socket_close($client);
                exit;
                break;
            default:
                // parent process 
                $this->process_stack[$pid] = true;
                break;
            }
        }
    }
}