GPix, the GPL Pixel Ad Script
Payment Processors Overview

This document explains the technical details of how the payment processors are
integrated into the application.

PAYPAL
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1. Configuration

Key paypal configuration items are:

  PayPal ID: email address of your merchant account. payments are made to this
  account.

  PayPal URL: URL for posing commands to PayPal. By default, this is the live
  PayPal site, but you can change it to the PayPal sandbox site for testing.

  Use IPN? If set, PayPal's Instant Payment Notification (IPN) process will be
  used, and a verification post will be made back to PayPal to verify the
  authenticity of the payment. Using IPN is strongly recommended, but it
  requires the cURL PHP extension, and it requires OpenSSL if a https:// IPN
  verification post is to be made (as recommended). If cURL is available but
  OpenSSL is not, a standard http:// IPN verification post can be made.

2. Process

PayPal payments are made using PayPal's "Buy-it-now" process. At the "Select
Payment Method" screen in the "Get Pixels" process, a form containing the
necessary variables is created to post to the PayPal URL. A payment_id number
is allocated and passed along with the PayPal form for tracking the payment
back into the database.

When the user selects "Pay with PayPal", the PayPal site handles the payment
and returns via a post to the paypal_return.php script.

Meanwhile, if IPN is enabled, PayPal will issue a separate post to the
paypal_ipn.php script. This script verifies the IPN with a post back to PayPal
and then records the IPN results in the payment record.

The paypal_return.php script waits up to 60 seconds for the IPN to be recorded
in the database (if IPN is enabled). It then marks the payment as complete and
invokes the next step in the get_pixels.php script, which stores the region
and links the completed payment to the region. If IPN is not used, the
paypal_return.php also marks the payment as verified.