Class 'yii\helpers\UnsetArrayValue' not found - why?

Question

Yii

Class 'yii\helpers\UnsetArrayValue' not found - why?

<?php

defined('YII_DEBUG') or define('YII_DEBUG', true);
defined('YII_ENV') or define('YII_ENV', 'dev');

require(__DIR__ . '/../vendor/autoload.php');
require(__DIR__ . '/../vendor/yiisoft/yii2/Yii.php');

new \yii\helpers\UnsetArrayValue();

Answer 1

because there is no such class in yii