Getting PayPal Confirmation after checkout in PHP, without a framework

I used to have a "Buy Now" button that would go through PayPal, and then, through the button's advanced option, I redirected the users to a "Success" page that would give the serial for the product they purchased.

Obviously, this page had to get confirmation from PayPal, through the variab开发者_Go百科les PayPal gave it, in order to know that a payment had actually been processed, and not only some person typing the page's URL in his browser to get a serial.

I did this only in PHP, without downloading any API, using, as far as I can remember, $_POST or $_SERVER variables from PayPal. There were a couple of nested "if"s, but in the end, it worked fine.

Now I've been Googling for an hour trying to find the code I used. All I find is framework downloads and bloated APIs, while I really only need to know these things:

• Did the order process?
• What is the email of the buyer?

I have a blank PHP file waiting for suggestions. In dummy-pseudo-code, it would go around the lines of:

<?php

    $serial = ...;

    if ($paypal_succeeded)
    {
        echo($serial);
    }

?>

Please don't tell me this is impossible, I know I already did it, I just lost my old code.

Thanks!

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I don't have any experience with this sort of thing, but google showed me a few possible relevant links here, here, and here.

The main page for this was https://www.paypal.com/ipn