How do you provide a unique url that expires or prevent a user from accessing...

How do you provide a unique url that expires or prevent a user from accessing your registration page directly?

I have a registration page that automatically lets you access the members page without ever being approved. This Reg form should only be accessible after you pay a paypal fee. But I think people are finding this link through search engines. What's the best way to stop this? .htaccess? or is there a way that the page won't load unless it's coming from paypal page? please, any advice is welcome. 

User authentication?

ok user authentication is good but anyone know of a script that has an admin cpanel which every user registered has to have it approved before they can access the members page?

wrong forum mate, should have been php and here is the solution to your problem, it's called "generating one-time url's" (that's a hard one  )

http://www.onlamp.com/pub/a/php/2002/12/05/one_time_URLs.html