Yii::$app->response->format = Response::FORMAT_JSON

 namespace api\components;

 use Yii;

 use yii\web\Response;

 class Controller extends \yii\web\Controller

 {

     protected $isLowIE;

     protected $callback;

     public function beforeAction($action)

     {

         $this->layout = false;

         $this->callback = Yii::$app->request->get('callback', false);

         $this->isLowIE = (boolean)Yii::$app->request->get('ie', false);

         if (!$this->isLowIE && !$this->callback) {

             Yii::$app->response->format = Response::FORMAT_JSON;

         }

         return parent::beforeAction($action);

     }

     /**

      * @param \yii\base\Action $action

      * @param mixed $result

      * @return mixed

      */

     public function afterAction($action, $result)

     {

         $result = parent::afterAction($action, $result);

         // your custom code here

         if ($this->isLowIE || $this->callback) {

             $result = json_encode($result);

             if ($this->callback) {

                 $result = $this->callback .'('. $result .')';

             }

         }

         return $result;

     }

 }