Fix IAP for iOS app

I need someone to take over an IOS app built with Ionic (backend PHP) and fix/build the following: 1) The previous payment was not accepted by apple and the current does not work. I need it to be converted to apple IAP so that I can publish the app to app store. 2) The app is a exam practice paper app, so basically the papers/profiles are store in backend (admin access via PHP). The app (ios), after the purchase, need to add the respective paper to the inventory of the user. 3) configuration of SSL for PHP app I can provide the source code and access to the backend where the PHP codes are and DB, however, i dont have any documentation on the PHP codes.