Why does a yii2 application output code from index.php?

Question

Yii

When I go to the url, I see just the code in the browser:

<?php

// comment out the following two lines when deployed to production
defined('YII_DEBUG') or define('YII_DEBUG', false);
defined('YII_ENV') or define('YII_ENV', 'prod');

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

$config = require(__DIR__ . '/../config/web.php');

(new yii\web\Application($config))->run();

although it works locally. Why do you think this is?