Opening a new file

Well Sir,
How do i open up a new .php3 file after i click a button.For ex. i have created a login page.If the user is not able to login as he was not previously registered , i want to take him to the regsiteration page after i echo the message that u have to register.I have linked php and MySQL for database.Would u b kind enough to tell me a way out?
Thanx
Shabana

How are you attempting to do this? Do you have a form with login/pass/submit? If so, just make the "action" of the "<form>" tag equal whatever page you check the user in (you could even use the same PHP page).

Nelson