Authentication is the act of verifying who a user is, and is the basis of the login process. Typically, authentication uses the combination of an identifier--a username or email address--and a password. The user submits these values through a form, and the application then compares the submitted information against that previously stored (e.g., upon registration).
In Yii, this entire process is performed semi-automatically, leaving the developer to merely implement [[yii\web\IdentityInterface]], the most important class in the authentication system. Typically, implementation of `IdentityInterface` is accomplished using the `User` model.
[advanced project template](https://github.com/yiisoft/yii2-app-advanced/blob/master/docs/guide/README.md). Below, only the interface methods are listed:
The `validateAuthKey` method just needs to compare the `$authKey` variable, passed as a parameter (itself retrieved from a cookie), with the value fetched from the database.