How to write a patch for a PHP application?

Question

PHP

Application on CodeIgniter. Supported by developers who release updates - archive with new version + database migration.
The application needs to work with some modifications. There is no mechanism for redefining/extending functionality in the application itself.
The task is to make changes and save them after the update.
I guess the best possible option is a script that will run after the update and make the necessary changes to the files.
CodeIgniter uses MVC, so what kind of changes should it make - modify/add class methods, modify views, i.e. in the right place to add a piece of view.

What tools can you recommend?

Answer 1

codeigniter.com/user_guide/extending/index.html
+ codeigniter.com/user_guide/concepts/services.html