After some time using an application, the need may arise to make a touch-up, add some functionality, or even change it drastically.
But it turns out that we no longer have contact with the original author, or he has completely abandoned the development and maintenance of such application, which is essential for our activity.
Who touches it now? I assure you that it is not easy to dive into the code of a program written by another and about whose development you do not have any documentation.
In these cases, you have to reverse engineer, to get to see the skeleton of the program, everything that covers it, and understand how each part works.
It is not easy at all, and requires knowledge, but extensive experience as a developer also helps a lot. Having participated to a lesser or greater extent in different projects allows you to identify which development strategies were used in general, and which techniques in particular.
If you need to tweak, improve, adapt or modernize that very fundamental web application, powered by PHP, maybe it can help you.