php 分销查找下级用户（无限查询）

//获取用户网体用户以下的用户
    public function getBottomUsers($uid,$uids=''){
        $userList = M('z_user')->field('uid,pre_uid')->where(array('pre_uid'=>$uid))->select();
        foreach ($userList as $key=>$value){
            $uids .= $value['uid'].',';
            $user = M('z_user')->field('uid,pre_uid')->where(array('pre_uid'=> $value['uid']))->select();
            if($user){
                $uids = $this->getBottomUsers($value['uid'],$uids);
            }
        }
        return $uids;
    }