public function executeListener($signal, $exit = true)
    {
        $return = null;

        if (isset($this->listeners[$signal])) {
            $return = call_user_func_array($this->listeners[$signal], [$signal]);
        }

        if (true === $exit) {
            // exit the application
            posix_kill(posix_getpid(), SIGKILL);
        }

        return $return;
    }