# Authentication ¶

Authentication is the process of verifying the identity of a user. It usually uses an identifier (e.g. a username or an email address) and a secret token (e.g. a password or an access token) to judge if the user is the one whom he claims as. Authentication is the basis of the login feature.

Yii provides an authentication framework which wires up various components to support login. To use this framework, you mainly need to do the following work:

- Configure the user application component;
- Create a class that implements the yii\web\IdentityInterface interface.

## Configuring yii\web\User ¶

The user application component manages the user authentication status. It requires you to specify an identity class which contains the actual authentication logic. In the following application configuration, the identity class for user is configured to be `app\models\User` whose implementation is explained in the next subsection:

    return [
        'components' => [
            'user' => [
                'identityClass' => 'app\models\User',
            ],
        ],
    ];

## Implementing yii\web\IdentityInterface ¶

The identity class must implement the yii\web\IdentityInterface which contains the following methods:

- findIdentity(): it looks for an instance of the identity class using the specified user ID. This method is used when you need to maintain the login status via session.
- findIdentityByAccessToken(): it looks for an instance of the identity class using the specified access token. This method is used when you need to authenticate a user by a single secret token (e.g. in a stateless RESTful application).
- getId(): it returns the ID of the user represented by this identity instance.
- getAuthKey(): it returns a key used to verify cookie-based login. The key is stored in the login cookie and will be later compared with the server-side version to make sure the login cookie is valid.
- validateAuthKey(): it implements the logic for verifying the cookie-based login key.

If a particular method is not needed, you may implement it with an empty body. For example, if your application is a pure stateless RESTful application, you would only need to implement findIdentityByAccessToken() and getId() while leaving all other methods with an empty body.

In the following example, an identity class is implemented as an Active Record class associated with the `user` database table.

    <?php

    use yii\db\ActiveRecord;
    use yii\web\IdentityInterface;

    class User extends ActiveRecord implements IdentityInterface
    {
        public static function tableName()
        {
            return 'user';
        }

        /**
         * Finds an identity by the given ID.
         *
         * @param string|int $id the ID to be looked for
         * @return IdentityInterface|null the identity object that matches the given ID.
         */
        public static function findIdentity($id)
        {
            return static::findOne($id);
        }

        /**
         * Finds an identity by the given token.
         *
         * @param string $token the token to be looked for
         * @return IdentityInterface|null the identity object that matches the given token.
         */
        public static function findIdentityByAccessToken($token, $type = null)
        {
            return static::findOne(['access_token' => $token]);
        }

        /**
         * @return int|string current user ID
         */
        public function getId()
        {
            return $this->id;
        }

        /**
         * @return string current user auth key
         */
        public function getAuthKey()
        {
            return $this->auth_key;
        }

        /**
         * @param string $authKey
         * @return bool if auth key is valid for current user
         */
        public function validateAuthKey($authKey)
        {
            return $this->getAuthKey() === $authKey;
        }
    }

As explained previously, you only need to implement `getAuthKey()` and `validateAuthKey()` if your application uses cookie-based login feature. In this case, you may use the following code to generate an auth key for each user and store it in the `user` table:

    class User extends ActiveRecord implements IdentityInterface
    {
        ......

        public function beforeSave($insert)
        {
            if (parent::beforeSave($insert)) {
                if ($this->isNewRecord) {
                    $this->auth_key = \Yii::$app->security->generateRandomString();
                }
                return true;
            }
            return false;
        }
    }

> Note: Do not confuse the `User` identity class with yii\web\User. The former is the class implementing the authentication logic. It is often implemented as an Active Record class associated with some persistent storage for storing the user credential information. The latter is an application component class responsible for managing the user authentication state.

## Using yii\web\User ¶

You mainly use yii\web\User in terms of the `user` application component.

You can detect the identity of the current user using the expression `Yii::$app->user->identity`. It returns an instance of the identity class representing the currently logged-in user, or `null` if the current user is not authenticated (meaning a guest). The following code shows how to retrieve other authentication-related information from yii\web\User:

    // the current user identity. `null` if the user is not authenticated.
    $identity = Yii::$app->user->identity;

    // the ID of the current user. `null` if the user not authenticated.
    $id = Yii::$app->user->id;

    // whether the current user is a guest (not authenticated)
    $isGuest = Yii::$app->user->isGuest;

To login a user, you may use the following code:

    // find a user identity with the specified username.
    // note that you may want to check the password if needed
    $identity = User::findOne(['username' => $username]);

    // logs in the user
    Yii::$app->user->login($identity);

The yii\web\User::login() method sets the identity of the current user to the yii\web\User. If session is enabled, it will keep the identity in the session so that the user authentication status is maintained throughout the whole session. If cookie-based login (i.e. "remember me" login) is enabled, it will also save the identity in a cookie so that the user authentication status can be recovered from the cookie as long as the cookie remains valid.

In order to enable cookie-based login, you need to configure yii\web\User::$enableAutoLogin to be `true` in the application configuration. You also need to provide a duration time parameter when calling the yii\web\User::login() method.

To logout a user, simply call

    Yii::$app->user->logout();

Note that logging out a user is only meaningful when session is enabled. The method will clean up the user authentication status from both memory and session. And by default, it will also destroy all user session data. If you want to keep the session data, you should call `Yii::$app->user->logout(false)`, instead.

## Authentication Events ¶

The yii\web\User class raises a few events during the login and logout processes.

- EVENT_BEFORE_LOGIN: raised at the beginning of yii\web\User::login(). If the event handler sets the isValid property of the event object to be `false`, the login process will be cancelled.
- EVENT_AFTER_LOGIN: raised after a successful login.
- EVENT_BEFORE_LOGOUT: raised at the beginning of yii\web\User::logout(). If the event handler sets the isValid property of the event object to be `false`, the logout process will be cancelled.
- EVENT_AFTER_LOGOUT: raised after a successful logout.

You may respond to these events to implement features such as login audit, online user statistics. For example, in the handler for EVENT_AFTER_LOGIN, you may record the login time and IP address in the `user` table.