递归处理层级展示

   public static function treeLevels($data,$pid="0",$html="--",$level=0){
        static $arr=[];
        foreach ($data as $key=>$val){
            if ($val['pid']==$pid){
                $val['html']=str_repeat($html,$level*2);
                $arr[]=$val;
                //递归 自己调用自己
                self::treeLevels($data,$val['id'],'--',$level+1);
            }
        }
        return $arr;

    }