Paypal IPN Handler for Cart

I'm using PayPal 'buy it now' buttons on my website to sell products. Because I keep track of the number of units in stock for each product in a MySQL database and I'd like the inventory tracking in the system to be automated, I am using PayPal's Instant Payment Notification functionality to let me know when a purchase has been completed. When Paypal notifies my handler that a valid purchase has been made, the script updates my MySQL database by subtracting '1' from the inventory of the product purchased.

I've attached my IPN PHP code below that works successfully with Paypal buy it now buttons (one purchase at a time).

My Question is: I would like to substitute 'buy it now' buttons with PayPal's 'add to cart' buttons so that customers can purchase more than one product at a time. I'm unsure how I have to alter my code below to let it loop through all items purchased and update my database accordingly. Any help would be greatly appreciated!

The Code:

    // Paypal POSTs HTML FORM variables to this page
// we must post all the variables back to paypal exactly unchanged and add an extra parameter cmd with value _notify-validate

// initialise a variable with the requried cmd parameter
$req = 'cmd=_notify-validate';

// go through each of the POSTed vars and add them to the variable
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";

// In a live application send it back to www.paypal.com
// but during development you will want to uswe the paypal sandbox

// comment out one of the following lines

$fp = fsockopen ('www.sandbox.paypal.com', 80, $errno, $errstr, 30);
//$fp = fsockopen ('www.paypal.com', 80, $errno, $errstr, 30);

// or use port 443 for an SSL connection
//$fp = fsockopen ('ssl://www.paypal.com', 44开发者_开发百科3, $errno, $errstr, 30);


if (!$fp) {
// HTTP ERROR
}
else
{
  fputs ($fp, $header . $req);
  while (!feof($fp)) {
    $res = fgets ($fp, 1024);
    if (strcmp ($res, "VERIFIED") == 0) {


      $item_name = stripslashes($_POST['item_name']);
      $item_number = $_POST['item_number'];
      $item_id = $_POST['custom'];  
      $payment_status = $_POST['payment_status'];
      $payment_amount = $_POST['mc_gross'];         //full amount of payment. payment_gross in US
      $payment_currency = $_POST['mc_currency'];
      $txn_id = $_POST['txn_id'];                   //unique transaction id
      $receiver_email = $_POST['receiver_email'];
      $payer_email = $_POST['payer_email'];
      $size = $_POST['option_selection1'];
      $item_id  = $_POST['item_id'];
      $business = $_POST['business'];



      if ($payment_status == 'Completed')  {  
// UPDATE THE DATABASE      


      }

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It would probably be easier for you to collect all item-ID's into an order-ID, then send that order-ID with your POST to PayPal. Once the IPN comes back with your details, check and calculate that the total sum of all items in the order-ID, matches the sum the IPN said was paid.

Query your database for the order-ID and get all the item-ID's connected to it, then decrease the stock number for each of the item-ID's.

Hope it helps. It's just one way of doing it!

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It is not clear what you are asking. I wrote a PayPal IPN listener for Website Payment Pro. I started by looking here PayPal Documentation: IPN Sample Code and then changing the code as I needed. Please add some more details to your question.