Yii2：找不到“Yii”类

I was learning Yii framework a few days ago. I facing with the error below:

Fatal error: Class 'Yii' not found in C:\xampp\htdocs\LoginForm\views\user\views\defaultegister.php on line 17

Try this first row:

use Yii;
//other use statements

and then:

Yii::$app->...

OR you can make it work this way to:

  \Yii:$app->...

Yii is present in the root folder of the project. So you need to tell the program where to locate it. Either write:

use Yii

at the top along with the other use statements, or put a backslash before Yii like: \Yii