Revision control (also known as source control or version control) allows us to keep track of the prerequisites:revision control" changes we make to our codebase. We can make a change, then commit it to source control, make more changes and commit them, and push our changes to other developers on the team. If we discover an error, we can revert to an earlier version of the codebase to a point where the error does not exist and start over.
If you are not using a source control tool like Git, Mercurial, Subversion, or some other revision control system, then that's the very first thing you need to put in place. Using source control will be a great benefit to you, even if you don't modernize your PHP application at all.
I prefer Mercurial in many ways, but I recognize that Git is more widely used, and as such I must recommend Git for new users of source control systems.
While it is beyond the scope of this book to discuss how to set up and use a source control system, there are...