How do I stop Users jumping ahead.

I'm building a site that allows users to sign up as a member then pay then enter their details to be advertised on my site. How do I stop people just going straight to the "enter details.php" page and bypassing the payment system?
I've considered session variables but I don't know if my payment processor will set them for me.
I've considered making an entry in the db when a payment is sucessful and then sending them to the "enter details" page but it seems a long way to go around the problem.
Does anyone have any ideas?

Thanks

No, Intead of making an entry in the db when a payment is successful, register a new session like this: session_register('payment')

Then when someone log into enter details page, you must check whether the session is registered(payment) in first few lines of the page. If not redirect user to payment page.

Hope it helped.

that is the right way to do it. when payment processor sends you a raport of payment, you enter it into the db.

when they come to the enter_details.php page, you check if payment has been made from a user with the same mail address (or some other unique ID field), and you let them enter details etc.

if not, you don't. that is the standard way to do it...

Thanks guys.

Nicky