How to correctly create a structure and algorithm for working with the Qiwi external API?

Question

PHP

Friends, in general, the thing is, I want to make it possible to sell a service on the site, I decided to choose the QIWI system because it is the simplest, but I can’t really find examples of working with the Qiwi API themselves.
The very essence of the question is that, in principle, we have a set of REST functions in Qiwi and how I imagine the algorithm of work
A user who decides to purchase a service on the site has the opportunity to pay for it through Qiwi.
On behalf of the user.
1) Service
2) Purchase
3) Invoicing to your wallet
4) Payment, payment made
How to keep records for the last 2 points, the user paid the invoice, how to track it?
If tracking is successful, how to change the status of access to a service or a restricted section of the site for a specific user in the system on the site.

Answer 1

3. At this stage, a payment form is formed on the site side, which contains the authorization data in the qiwi payment system and the required parameters order amount, order currency, order comment, site page in case of successful payment, site page in case of a payment error, etc. . These page addresses can be specified in the personal account of the payment system.
4. Upon successful payment of the order on the side of the payment system, the return to the page of the site of successful payment occurs, the relevant information is displayed on the screen, the site registers the receipt of payment (money is received on the client’s account) and then funds are debited for the amount of the order.

Answer 2

The Qiwi REST API provides a notification about successful payment of an invoice, or about cancellation of an invoice by a user (there is still expired - the invoice is overdue).
In the merchant settings in your personal account, in my opinion, you can change the url to which notifications will be sent in a certain format (essentially a regular POST request).
Everything is quite simply implemented there, the documentation is enough without examples.