Redirect URL

Hi I have form in file register.php. The action of this form is the same - register.php the form validation script is included in this file like include("mail.php") In mail.php there is two sections - one is working if user didn't complete form properly another if everything okay. So, I need form to show the different page (let's say success.htm) if everything was filled right. I include the following in the mail.php file if everything is okay: header("Location: http://test.my/success.htm"); But it doesn't work even though all the files are in the same directory. it shows: Warning: Cannot modify header information - headers already sent by (output started at z:\home\test.my\www\register.php:5) in z:\home\test.my\www\mail.php on line 41 What is wrong? Yury