protected function getFds()
    {
        $fds = array_filter($this->to, function ($value) {
            return is_integer($value);
        });
        $rooms = array_diff($this->to, $fds);

        foreach ($rooms as $room) {
            $clients = $this->room->getClients($room);
            // fallback fd with wrong type back to fds array
            if (empty($clients) && is_numeric($room)) {
                $fds[] = $room;
            } else {
                $fds = array_merge($fds, $clients);
            }
        }

        return array_values(array_unique($fds));
    }