We use PayPal's IPN for what you are talking about. Basically you set up a listener script in php (see code posted below) on your server. Then when a sale is made, paypal calls that script and it does what you program it to do, such as email you or add info to a database.
Here is PayPal's generic php listener script which is a good place to start.
<?php
// read the post from PayPal system and add 'cmd'
$req = 'cmd=' . urlencode('_notify-validate');
foreach ($_POST as $key => $value) {
$value = urlencode(stripslashes($value));
$req .= "&$key=$value";
}
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://www.paypal.com/cgi-bin/webscr');
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $req);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Host: www.paypal.com'));
$res = curl_exec($ch);
curl_close($ch);
// assign posted variables to local variables
$item_name = $_POST['item_name'];
$item_number = $_POST['item_number'];
$payment_status = $_POST['payment_status'];
$payment_amount = $_POST['mc_gross'];
$payment_currency = $_POST['mc_currency'];
$txn_id = $_POST['txn_id'];
$receiver_email = $_POST['receiver_email'];
$payer_email = $_POST['payer_email'];
if (strcmp ($res, "VERIFIED") == 0) {
// check the payment_status is Completed
// check that txn_id has not been previously processed
// check that receiver_email is your Primary PayPal email
// check that payment_amount/payment_currency are correct
// process payment
}
else if (strcmp ($res, "INVALID") == 0) {
// log for manual investigation
}
?> As you can see you will need CURL for php installed on your server. If the host your planning on using doesn't offer that there are plenty that do.
