Yii2 authKey what's the purpose?

Question

My user authorization works fine without authKey. I don't get how should I use it. But it's somehow more secure to use it as it stands in documentation. I've implemented this methods in User ActiveRecord class.

public function generateAuthKey()
{
    $this->auth_key = Yii::$app->security->generateRandomString();
}

public function validateAuthKey($authKey)
{
    return $this->authKey === $authKey;
}

I even save it to user table on user creation. But I know that validateAuthKey is never used. I'm using sessions and only PHPSESSID sent to user. Do I have to set authKey cookie manually? What are the advantages of this? Why can't I authorize user just by PHPSESSID. It's already stored in session table. Session configuration:

'session' => [
    'class' => 'yii\web\DbSession',
    'sessionTable' => 'session',
],

Answer 1

There’s an answer already in Yii2: Why is the auth key in class User?

See the following source code (vendor\yiisoft\yii2\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__);
        }
    }