php 无限极递归

public function recursion($list, $newArr=[], $pid = '0',$level = 0){
    $newArr = []; // 新数组用于存放数据
    if (!empty($list)) {
        foreach ($list as $k => &$v) {
            if ($v['pid'] == $pid) { // 如果父ID与pid相同则为父级数组
               $v['level'] = $level; // 层级
               $v['child'] = $this->recursion($list, $newArr, $v['id'],$level+1); // 递归处理数据
               $newArr[] = $v; // 新数据放入数组准备返回
            }
        }
    }
    return $newArr;
    // 使用方法
    /*$city = DB::table('citylist')->get();
    $arr  = json_decode(json_encode($city),true);
    print_r($this->recursion($arr));*/
}