While convenient, yii\rest\ActiveController doesn't solve every problem with creating APIs. When not using yii\rest\ActiveController, you'll want to extend your controller classes from yii\rest\Controller in order to take full advantage the built-in REST API defaults implemented by yii\rest\Controller. The following sections illustrate some additional information on creating custom API controllers.
There are several way in which we can think about custom API controllers in Yii2. The easiest way to think about passing data to our clients is to bypass the view portion of our MVC model and directly return data from our controllers. For example, if we were to create a new controller called SiteController within our controller's namespace, we could directly return data from our newly created controller, as follows:
<?php
namespace app\controllers;
use Yii;
class SiteController extends \yii\rest\Controller
{
public function actionIndex()
{
...