confused when using createController and render

<<<<Yii 2.0>>>>

in vendor/yiisoft/yii2/Yii.php ,I add a function like




    static private function mytest(){


        $cc = Yii::$app->createController('site/show');

        $cc[0]->runAction('show');


        //cc[0]->runAction('show')->render("/site/show");

        //$cc[0]->render("/site/show");


    }



while in frontend/controllers/SiteContoller.php,action is such defined




        public function actionShow(){

                return $this->render('show');

        }



and in frontend/themes/default/site/show.php,it,s just,




<?php echo "ssssssss";?>



question is ,when call,Yii::mytest,i just got a white page with nothing on it!

is there anything wrong on the function mytest()??