This text discusses the process of updating a legacy PHP application with its own user data store to use a modern authentication system like FusionAuth. The application, called "The ATM," has a homegrown auth system that is hard to integrate with other systems and causes maintenance headaches for developers. To update this application, the author sets up an instance of FusionAuth, configures it to correspond to the legacy PHP application, and then updates the PHP application to use OAuth and OIDC instead of its own local datastore. The updated code redirects users to the FusionAuth hosted login pages when they click "Login," and after authentication, executes a callback function that retrieves user data from FusionAuth and stores it in the session. This change enables the application to authenticate using a centralized auth system, offloading complexities and security risks to FusionAuth while still allowing access control based on user roles.