如何通过php循环和递归实现无限级分类

<!--?php
header("content-type:text/html;charset=utf-8");
$pdo = new PDO("mysql:host=127.0.0.1;dbname=ninthexam","root","root");
$rs = $pdo ---> query("select * from city");
while($row = $rs -> fetch()){
    $data[] = $row;
} 
//循环实现分类
foreach($data as $k=>$v)
{
    if($v['parint_id'] == 0)
    {
        $tmp[] = $v;
        foreach($data as $key=>$value)
        {
            if($v['city_id'] == $value['parint_id'])
            {
                $tmp[] = $value;
            }
        }
    }
}
echo "
"; print_r($tmp);

    function digui($data,$parint_id = 0 ,$cengji = 0)
       {
           //使用静态定义
           static $tmp = array();
           foreach($data as $k=>$v)
           {
               //判断如果“层级ID==自增ID”
               if($v['parint_id'] == $parint_id)
               {
                   $v['cengji'] = $cengji;
                   $tmp[] = $v;
                   digui($data,$v['city_id'],$cengji+1);
               }
           }
           return $tmp;
       }
print_r(digui($data,$parint_id = 0 ,$cengji = 0));