redirection rather than include()

This is a PHP newbie trying to figure something out. I am creating a web-app which begins with a login page and goes through the typical user/pass validation and upon success moves to a frameset where the application will run. The action of the login form is act_login.php. In that script I validate the user/pass and, upon validation, include() the frameset script. However, I would rather redirect the browser to the frameset script. In other words I want to progression to go like this... login form --> successful login (determined by act_login.php) --> redirect browser to app frameset This scenario is probably swimming in technicalities, and I'm probably just fine using the include() statement. However, I guess my overall question is... is there a way to redirect the browser to a new URL (eg. php script) inside something as simple as an if/then statement? Thanks!