src/web/User.php
protected function loginByCookie()
    {
        $value = Yii::$app->getRequest()->getCookies()->getValue($this->identityCookie['name']);
        if ($value === null) {
            return;
        }

        $data = json_decode($value, true);
        if (count($data) !== 3 || !isset($data[0], $data[1], $data[2])) {
            return;
        }

        list ($id, $authKey, $duration) = $data;
        /* @var $class IdentityInterface */
        $class = $this->identityClass;
        $identity = $class::findIdentity($id);
        if ($identity === null) {
            return;
        } elseif (!$identity instanceof IdentityInterface) {
            throw new InvalidValueException("$class::findIdentity() must return an object implementing IdentityInterface.");
        }

        if ($identity->validateAuthKey($authKey)) {
            if ($this->beforeLogin($identity, true, $duration)) {
                $this->switchIdentity($identity, $this->autoRenewCookie ? $duration : 0);
                $ip = Yii::$app->getRequest()->getUserIP();
                Yii::info("User '$id' logged in from $ip via cookie.", __METHOD__);
                $this->afterLogin($identity, true, $duration);
            }
        } else {
            Yii::warning("Invalid auth key attempted for user '$id': $authKey", __METHOD__);
        }
    }
protected function loginByCookie()
    {
        $value = Yii::$app->getRequest()->getCookies()->getValue($this->identityCookie['name']);
        if ($value === null) {
            return;
        }

        $data = json_decode($value, true);
        if (count($data) !== 3 || !isset($data[0], $data[1], $data[2])) {
            return;
        }

        list ($id, $authKey, $duration) = $data;
        /* @var $class IdentityInterface */
        $class = $this->identityClass;
        $identity = $class::findIdentity($id);
        if ($identity === null) {
            return;
        } elseif (!$identity instanceof IdentityInterface) {
            throw new InvalidValueException("$class::findIdentity() must return an object implementing IdentityInterface.");
        }

        if ($identity->validateAuthKey($authKey)) {
            if ($this->beforeLogin($identity, true, $duration)) {
                $this->switchIdentity($identity, $this->autoRenewCookie ? $duration : 0);
                $ip = Yii::$app->getRequest()->getUserIP();
                Yii::info("User '$id' logged in from $ip via cookie.", __METHOD__);
                $this->afterLogin($identity, true, $duration);
            }
        } else {
            Yii::warning("Invalid auth key attempted for user '$id': $authKey", __METHOD__);
        }
    }
Logs in a user by cookie.

This method attempts to log in a user using the ID and authKey information
provided by the [[identityCookie|identity cookie]].