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: 
"using a transactionHash that could be calculated based on the customer's email and the product unique id."

As soon as the algorithm gets known you system will break down. My recommendation is a "secure" aka cryptographic PRNG + some lookup table.
include a hash param which value is calculated based on several parameters. for example, for your url o would calculate the hash like this:

```
$uniqueKeyString="some random characters";
transactionHash=md5("domain.com".$productId.time().$uniqueKeyString);

```

where `$uniqueKeyString` is a secret value (some random integer) only you know.
than, when a request will come to your servers, you can simply calculate the hash string yourself and compare it with the `transactionHash` of the request whether it is the same.