http://www.micahcarrick.com/php-paypal-ipn-integration-class.html
y ya la pude integrar a mis pruebas comprando/pagando un solo articulo, el problema me viene cuando quiero agregar mas de un articulo, no lo logro hacer.
Este es el codigo de mi clase:
Código PHP:
<?php
require_once('paypal.class.php'); // include the class file
$dir_sandbox='https://www.sandbox.paypal.com/cgi-bin/webscr'; // testing paypal url
$dir='https://www.paypal.com/cgi-bin/webscr';
$p = new paypal_class; // initiate an instance of the class
$p->paypal_url = $dir_sandbox;
// setup a variable for this script (ie: 'http://www.micahcarrick.com/paypal.php')
$this_script = 'http://'.$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF'];
// if there is not action variable, set the default action of 'process'
if (empty($_GET['action'])) $_GET['action'] = 'process';
switch ($_GET['action']) {
case 'process': // Process and order...
$p->add_field('business', '[email protected]');
$p->add_field('return', $this_script.'?action=success');
$p->add_field('cancel_return', $this_script.'?action=cancel');
$p->add_field('notify_url', $this_script.'?action=ipn');
$p->add_field('item_name', 'prueba de pago');
$p->add_field('amount', '2.00');
$p->add_field('currency_code', 'EUR');
$p->add_field('quantity', '2');
$p->add_field('item_number', '001');
$p->submit_paypal_post(); // submit the fields to paypal
//$p->dump_fields(); // for debugging, output a table of all the fields
break;
case 'success': // Order was successful...
// This is where you would probably want to thank the user for their order
// or what have you. The order information at this point is in POST
// variables. However, you don't want to "process" the order until you
// get validation from the IPN. That's where you would have the code to
// email an admin, update the database with payment status, activate a
// membership, etc.
echo "<html><head><title>Success</title></head><body><h3>Thank you for your order.</h3>";
foreach ($_POST as $key => $value) {
echo "$key: $value<br>";
}
echo "</body></html>";
// You could also simply re-direct them to another page, or your own
// order status page which presents the user with the status of their
// order based on a database (which can be modified with the IPN code
// below).
break;
case 'cancel': // Order was canceled...
// The order was canceled before being completed.
echo "<html><head><title>Canceled</title></head><body><h3>The order was canceled.</h3>";
echo "</body></html>";
break;
case 'ipn': // Paypal is calling page for IPN validation...
// It's important to remember that paypal calling this script. There
// is no output here. This is where you validate the IPN data and if it's
// valid, update your database to signify that the user has payed. If
// you try and use an echo or printf function here it's not going to do you
// a bit of good. This is on the "backend". That is why, by default, the
// class logs all IPN data to a text file.
if ($p->validate_ipn()) {
// Payment has been recieved and IPN is verified. This is where you
// update your database to activate or process the order, or setup
// the database with the user's order details, email an administrator,
// etc. You can access a slew of information via the ipn_data() array.
// Check the paypal documentation for specifics on what information
// is available in the IPN POST variables. Basically, all the POST vars
// which paypal sends, which we send back for validation, are now stored
// in the ipn_data() array.
$user="user";
$servidor="localhost";
$pass="pass";
$db="db";
$connection=mysql_connect ($servidor, $user, $pass);
if (!$connection) {
die('Not connected : ' . mysql_error());
}
// Set the active mySQL database
$db_selected = mysql_select_db($db, $connection);
if (!$db_selected) {
die ('Can\'t use db : ' . mysql_error());
}
if(mysql_query("insert into pagado (id,pagado) values(0,0)")){
echo "exito";
}else{
echo "error";
}
}
break;
}
?> 
