Айрат Хисямов

public $serializer ='frontend\modules\v2\component\CustomSerializer',

    public function actionSpecialFoApi()
    {

     if(rest_responce == true) {
        /** @var \frontend\modules\v2\component\ApicoSerializer $serializer */
        return Yii::createObject($this->serializer)->serialize($data);
        return; //возможно это может надо???
    }
}

Тут написано
yii\rest\Serializer
Serializes the given data into a format that can be easily turned into other formats.
You may override this method to support more object types.

Код не тестировал, сори)