code: protected function sendIdentityCookie($identity, $duration)
      {
          $cookie = new Cookie($this->identityCookie);
          $cookie->value = json_encode([
              $identity->getId(),
              $identity->getAuthKey(),
              $duration,
          ], JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
          $cookie->expire = time() + $duration;
          Yii::$app->getResponse()->getCookies()->add($cookie);
      }
docs: Sends an identity cookie.
  This method is used when [[enableAutoLogin]] is true.
  It saves [[id]], [[IdentityInterface::getAuthKey()|auth key]], and the duration of cookie-based login
  information in the cookie.
  @param IdentityInterface $identity
  @param integer $duration number of seconds that the user can remain in logged-in status.
  @see loginByCookie()