Unitpay payment with digital key?

Question

PHP

How does unitpay work?
We give the user a link with the required parameters and a digital key (in which the same parameters are encrypted). When the user clicks on the link, payment is opened on unitpay. But where is the digital key checked then?
I thought that before payment, it is formed again from the parameters and compared with what was in the url. If they match, the payment opens.
Is it self-made?
Let's say with webmoney: if the payment has passed, it executes the script, that is, something is charged to the user.
There is no such thing in unitpay .. Or is there?