index.php is the first file that loaded when we run an application using the Yii application. It will create a new object of new yii\web\Application and start the application.require(__DIR__ . '/../vendor/autoload.php');
require(__DIR__ . '/../vendor/yiisoft/yii2/Yii.php');
// load application configuration
$config = require(__DIR__ . '/../config/web.php');
// instantiate and configure the application
(new yii\web\Application($config))->run(); ​