无限极分类

    无限级分类，在做权限时可以将层级展现出来，

public function privilege(){
        $data = Db::table('privilege')->select();
        return $this->tree($data,$parent_id=0,$level=0);
    }

    public function tree($arr,$parent_id,$level){
        static $tree=[];
        foreach($arr as $key =>$v){
            if($v['parent_id'] == $parent_id){
                $tree[]=[
                    'privilege_id'=>$v['privilege_id'],
                    'privilege_name'=>$v['privilege_name'],
                    'level' => $level,
                    'parent_id'=>$parent_id
                ];
                $this->tree($arr,$v['privilege_id'],$level+1);
            }
        }
        return view("privilege_add",['arr'=>$tree]);
    }

             添加时的代码
             <tr>
                <td class="label">父级菜单</td>
                <td>
                    <select name="parent_id" >
                        <option value="0">顶级权限</option>
                        {volist name="arr" id="vo"}
                        <option value="{$vo.privilege_id}">
                            <a href="">{php} echo str_repeat('&nbsp&nbsp',$vo['level']*2){/php}{$vo.privilege_name}</a>
                        </option>
                        {/volist}
                    </select>
                </td>
            </tr>