ferower

  public static function getSubTopicIdsOfTopic($topicId, $topics) 
  {
    $list = array();
    foreach ($topics as $key => $topic) {
      if ($topic->parentId == $topicId) {
        $list[] = $topic->id;
        $list = array_merge($list, self::getSubTopicIdsOfTopic($topic->id, $topics));
      }
    }
    return $list;