How to refactor files into a couple of thousand lines?

Question

PHP

It was a simple 3 page website. Now it is a complex site on 3 pages. Bek has grown from a couple of php files into a hundred complex files in three years. Integration with srm, sms, telephony, instant messengers and hell. Added agency and partner cabinets. A bunch of webhooks, widgets...
In general, there are many files with complex logic on the back. some of them are 500 line switch statements. Which side to approach the refractor of such files?

Answer 1

1. You open a file.
2. You read the code.
3. Trying to understand.
4. Write tests
5. Refactor.

Answer 2

Decomposition .

Answer 3

It is worth starting with the study of refactoring methods.