Simple script for posting a valid response back to Paypal service after a sale is completed using the Paypal Instant Checkout payment method.

<?php
// read the post from PayPal system and add 'cmd'
$req = 'cmd=_notify-validate';
foreach ($_POST as $key => $value) {
$value = urlencode(stripslashes($value));
$req .= "&$key=$value";
}
// post back to PayPal system to validate
$header .= "POST /cgi-bin/webscr HTTP/1.0\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Content-Length: " . strlen($req) . "\r\n\r\n";
$fp = fsockopen ('ssl://www.paypal.com', 443, $errno, $errstr, 30);
// 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      = $_POST['payer_email'];
if (!$fp) {
// HTTP ERROR
} else {
fputs ($fp, $header . $req);
while (!feof($fp)) {
$res = fgets ($fp, 1024);
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
}
}
fclose ($fp);
}

I grabbed this script from Paypal Docs... I never can find the info I need w/in their site so I'm posting here for reference. https://www.paypal.com/us/cgi-bin/webscr?cmd=p/pdn/ipn-codesamples-pop-outside#php