App\Models\Forum\Forum::allSubforums PHP Method

    public function allSubforums($forum_ids = null, $new_forum_ids = null)
    {
        if ($forum_ids === null) {
            $forum_ids = $new_forum_ids = [$this->forum_id];
        }
        $new_forum_ids = model_pluck(static::whereIn('parent_id', $new_forum_ids), 'forum_id');
        $new_forum_ids = array_map('intval', $new_forum_ids);
        $forum_ids = array_merge($forum_ids, $new_forum_ids);
        if (count($new_forum_ids) === 0) {
            return $forum_ids;
        } else {
            return $this->allSubforums($forum_ids, $new_forum_ids);
        }
    }