How to Process PayPal Instant Payment Notification (IPN) with PHP
I have worked on numerous sites that had a need to collect either a one-time payment or establish a recurring (subscription) payment. I always recommend PayPal for collecting payments. PayPal not only offers the ability to make payment with PayPal but also allows non-PayPal buyers to pay with a debit or credit card. There is no charge to join PayPal and no monthly fee to use their service. There is only a charge when you receive a payment. Most clients I work with like not having to pay until you use it.