INSERT INTO `tree` 
(`id`, `parent_id`, `name`) 
VALUES 
(1, 0, 'A'), 
(2, 0, 'B'), 
(3, 1, 'a'), 
(4, 3, 'aa'), 
(5, 2, 'b'), 
(6, 4, 'aaa');

class tree { 
//访问index查看树形结构 
public function actionIndex () { 
$data = self::getTree(); 
//为了方便测试，我们这里以json格式输出 
\Yii::$app->response->format = \yii\web\Response::FORMAT_JSON; 
return $data; 
} 
//获取树 
public static function getTree () { 
//这里我们直接获取所有的数据，然后通过程序进行处理 
//在无限极分类中最忌讳的是对数据库进行层层操作，也就很容易造成内存溢出 
//最后电脑死机的结果 
$data = static::find()->all(); 
return self::_generateTree($data); 
} 
//生成树 
private static function _generateTree ($data, $pid = 0) { 
$tree = []; 
if ($data && is_array($data)) { 
foreach($data as $v) { 
if($v['parent_id'] == $pid) { 
$tree[] = [ 
'id' => $v['id'], 
'name' => $v['name'], 
'parent_id' => $v['parent_id'], 
'children' => self::_generateTree($data, $v['id']), 
]; 
} 
} 
} 
return $tree; 
} 
}