use Symfony\Component\HttpFoundation\Session\SessionInterface; public function register(User $user, SessionInterface $session) { // register user $session->getFlashBag()->add('success', 'You have successfully registered!'); $response = new RedirectResponse('homepage.php'); return $response->withSession($session); }In this code, we first register the user and then add a success flash message to the session using the `add` method of the `FlashBag` object. We then create a `RedirectResponse` object that redirects the user to the homepage and returns this response with the session, which includes the flash message. Overall, the PHP Session addMessageAfterRedirect is a useful method that simplifies the process of adding flash messages to sessions and displaying them after a redirect. It can be found in various PHP package libraries, such as the Symfony PHP framework.