<select class="sel">
    {volist name="arr" id="v"}
    <option value="{$v.id}">{$v.name}</option>
    {/volist}
</select>
<script>
        $(document).on('change','.sel',function(){
            //定義一個id
            var id=$(this).val();

            var obj=$(this);
//            alert(obj);die;
            $.get("{:url('Index/pid')}",{id:id},function(data){
                //刪除後面所有的
                obj.nextAll().remove();
                //轉化為內建物件
                var json=jQuery.parseJSON(data);
                //判斷他沒有找到的話，就false掉
                if(json==''){
                    return false;
                }
                var sel="<select class='sel'>";
                $(json).each(function(key,val){
                    sel+="<option value='"+val.id+"'>"+val.name+"</option>"
                });
                sel+="</select>";
                //後面追加
                obj.after(sel);
            })
        })
</script>
 

<?php
namespace app\index\controller;
use app\index\model\Index as u;
use think\Controller;
use think\Db;
use think\Request;

class index extends Controller{
    public function index(){
       $res=Db::table('attr')->where('pid=0')->select();
        return view('index',['arr'=>$res]);
    }
    public function pid(){
        $id=Request::instance()->get('id');
        $arr=Db::table('attr')->where('pid',$id)->select();
        return json_encode($arr);
    }
}