plz its needful for my carrer .i want script / code for the following.. love if you can

1.There is a button on my webpage.
2. ON clicking it it redirects to paypal.
3.After payment it had to go to a form which will be filled by user.{page must be only accesssible by successful payment no direct accesss}
4.Sorry but cannot use http refer as it can be spoofed.
5.The form can be filled by user once. After submit session expire.
6.if you want to fill form once again pay.