MajAfy

Hello, I wrote my owned Payment Gateway, every thing is ok and after the payment was done the page will redirected to http://www.domain.com/modules/gateway/callback/my_gateway.php and in this page I add the fund to customer account and after that I redirected the page to invoice page, until this step everything is OK some times we should show an error message to user, how can I do this ? for example I want show the successful or unsuccessful to my customer . Regards,