Question: I have a wordpress site at a Ubuntu 12.04 server with Apache installed as the web server. I would like to temporarily remove it from my website and enable another website (not wordpress, just plain HTML,PHP). How can I do that?

thanks

Answer: 
If you target PayPal only, why don't you check Instant Payment Notification Guide?
<https://www.x.com/sites/default/files/ipnguide.pdf>

I didn't use Paypal before, but it seems this solves your problem.

Create table:
| product\_id (unique ID of you product) | varchar transaction\_hash |

In this sample code (PHP example):
<https://www.x.com/developers/PayPal/documentation-tools/code-sample/216623>

After validating that the payment is correct, insert product ID and verify\_sign( value from paypal POST data) in the table. and Give the user a URL with with product ID and verify\_sign.
You can create a random id for a user at any given time you want, maybe even using some truely random generators out on the web.

BUT what you should do is make it UNIQUE for a specific amount of time, perhaps with a simple database structure, maybe storing informations into files on your server, that will be deleted by the same script as soon as they're read once, depends on your needs.

So whenever a user generates such unique ID he can access that information for either a certain period of time, or exactly once.

Using say random.org's random byte function you can generate a string like:
6f0d47cf3432d4015e0e798641191bf0e8e0b90b00df23181bcb3401a0dad43d85be711343c3baa9
Which is nearly impossible to guess even if someone else knows a productID AND the emailadress of said customer