There are several PHP scripts and classes to process PayPal payments using their native IPN (Internet payment notification) feature. Because the whole process is based on the data you need to send via a web form to the PayPal payment processor these script look very similar.
The payment / notification process is shown via the following graphic:

Inside the form there are several required values to process a payment. PayPal gives the advice to post them all to get everything working. The following variables get some special attention:

Quote:

business

Quote:

= your PayPal email address cmd = single payments or subscription service (_xclick or _xclick-subscriptions) return = the URL where the buyer get back after the payment is processed cancel_return = the URL where the buyer get back if he has cancelled the payment notify_url = the location where your IPN script is located rm = how you need the data submitted from PayPal to your IPN script (1=get, 2=post) currency_code = the currency you accept for your payment lc = the country version of PayPal where your buyer is send to

There are much more variables, but we think that the other variables (product, order and shipment information) speak for themselves. Find a complete form provided with the example files.
To run some IPN enabled payment process we need a small script which will double check if the data which is send to the IPN script is valid according the data which is stored on the PayPal server. This feature is very important if your e-commerce accepts automatic payments.
The following code is able to check if the payment is valid against the PayPal server. Use this test to decide if the payment is valid or not.

HTML Code:

<?php
$url = ‘https://www.paypal.com/cgi-bin/webscr’;
$postdata = ”;
foreach($_POST as $i =&gt; $v) {
$postdata .= $i.‘=’.urlencode($v).‘&amp;’;
}
$postdata .= ‘cmd=_notify-validate’; $web = parse_url($url);
if ($web[’scheme’] == ‘https’) {
$web[‘port’] = 443;
$ssl = ’ssl://’;
} else {
$web[‘port’] = 80;
$ssl = ”;
}
$fp = @fsockopen($ssl.$web[‘host’], $web[‘port’], $errnum, $errstr, 30); if (!
$fp) {
echo $errnum.‘: ’.$errstr;
} else {
fputs($fp, “POST ”.$web[‘path’].“ HTTP/1.1rn”);
fputs($fp, “Host: ”.$web[‘host’].“rn”);
fputs($fp, “Content-type: application/x-www-form-urlencodedrn”);
fputs($fp, “Content-length: ”.strlen($postdata).“rn”);
fputs($fp, “Connection: closernrn”);
fputs($fp, $postdata . “rnrn”); while(!
feof($fp)) {
$info[] = @fgets($fp, 1024);
}
fclose($fp);
$info = implode(‘,’, $info);
if (eregi(‘VERIFIED’, $info)) {
// yes valid, f.e. change payment status
} else {
// invalid, log error or something
}
}

?>

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