Paypal Intergration ?

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I know many ppl have probably seen this kind of request before, but here it is again im sorry.

I have been thinking lately about how to intergrate paypal with vbulletin registration (not expiration of membership - yet!)

Possible have the exit page of paypal once the user has paid point to say preregister.php, in this script the user would enter their email address and click on a submit button, the script would then send a code to this email address (something like the Current activation code, The user will have to enter this into the register script along with their other details.

I realise many ppl would be saying to themsleves "but the user could just go back to "preregister.php" and get a new code. this is true, but wot about if we have something like a referer check.
I.e if the user neve came from "www.paypal.com" then he gets redirected to a page of ur choice. something along the lines of

<?php
$ref_domain = getenv('HTTP_REFERER');
$ref_domain = parse_url($ref_domain);

if ($ref_domain['host'] == "www.paypal.com")
etc...

Then the mail script follows.

The code that is sent could/will be active for 24 hrs then deleted from the database.

Any comments? Opinions are welcome and im open for critisicism.

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