Validate that IPN call is from PayPal?

Question

How can I validate that a PayPal IPN POST request to my specified notifyURL is indeed coming from PayPal?

I don\'t mean comparing the data to what I sent earlier, bu

Answer 1

This is what I use:

if (preg_match('~^(?:.+[.])?paypal[.]com$~', gethostbyaddr($_SERVER['REMOTE_ADDR'])) > 0)
{
    // came from paypal.com (unless your server got r00ted)
}