How to write WeChat payment? What should be written on the PHP side?

**Our project is an app. Now we need to write a native WeChat payment interface.
How to write the PHP backend?
Which one of the official documents provided by WeChat should be read (didn’t understand)?
Is that a document used in the backend?
(I am a newbie and have never written a payment interface. If there is a development tutorial or something, please provide a link. Thank you!!!)**

This is the development, how to write the php backend?

Why does this have an SDK for PHP?

And the app payment I make does not have a PHP SDK?