How to check the payment and credit the user's balance through ishop.qiwi.com?

Question

PHP

I made a form on my site, people pay everything normally, the money comes in, but how to automatically process the payment and credit funds to the user's balance on the site?
In the Qiwi service, I found such a function as the REST protocol, where you can set up a payment notification to the specified address.
Does anyone have a code example with this function?