yii2 输出json的方法

public function actionAjax()
{
    if(isset(Yii::$app->request->post('test'))){
        $test = "Ajax Worked!";
        // do your query stuff here
    }else{
        $test = "Ajax failed";
        // do your query stuff here
    }

    // return Json    
    return yiihelpersJson::encode($test);
}