Modify login function to authenticate PHP webapp via API from separate authentication domain -- 2

We have an existing web app that has been a standalone project. We have since built other modules residing on separate domains and have decided to convert to a centralized user authentication via restAPI.

(API endpoints for: status, sign-in, signup, and lost password)

We need to modify the user management functions on our old webapp to use the API on the new domain for those functions instead.