递归（无限极……）

代码：

public function selMenu($parent_id = 0,$lv = 0){
    $data = Db::name('menu') -> order("id asc") -> select();
    static $arr = [];
    foreach($data as $k => $menu){
        if($menu['parent_id'] == $parent_id){
            $menu['menu_name'] = str_repeat('  ',$lv).$menu['menu_name'];
            $arr[] = $menu;
            $this -> selMenu($menu['id'],$lv+1);
        }
    }
    return $arr;
}