This guide takes you from "I integrate HyperPay/Paymob/Stripe webhooks" to "I can design the payment infrastructure of a fintech company." It's written assuming PHP 8.x + Laravel as the implementation stack, since that's your strongest base.
1. Business Requirements
What problem does a Payment System actually solve?
A naive view: "take money from customer, give to merchant." The real problem is reconciling three asynchronous, unreliable sources of truth — your application state, the payment gateway's state, and the bank/card network's state — while guaranteeing money is never lost, never duplicated, and every movement is auditable for accounting and legal purposes.
Everything else (idempotency, state machines, webhooks, reconciliation) exists because of one fact: payment confirmation is asynchronous and can fail silently, arrive late, arrive twice, or arrive out of order.
