General question...

Ok I think I know what I need to learn... how can I do a redirect based on whether or not a user came from a certain page? For example... if a user comes from a page called confirm.php then it takes them to a page called thanks.php and if the user came from a different server or typed the url directly then it would redirect the user to the index.php page? Thanks in advance... Jas P.S. A tutorial on this would definately prevent me from posting the same question again. =)