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.
Using a hash to access some stored information without the need of logging in isn't a bad idea. BUT that hash should not be generated based on already known data like IDs, email-address or similar data that could be known or guessed by any user.

Instead it is necessary to randomly generate a long enough hash thats value couldn't be guessed or generated out of any known data.
The already mentioned byte function from random.org could be a good choice for that.