All good advice for the average PHP developer, but I think it is overkill for someone that is starting with PHP just to keep alive a very old app.
Adding tests for an app with no documentation requires a lot of effort.
Adding tests for an app with no documentation requires a lot of effort.
code tests yes. Assuming it's a web app, probably not too hard. Get something that records the browser interactions, and do the common tasks people do (log in, click links, make a report, etc). Having general things like that automated to ensure you can run them repeatedly to make sure basic stuff didn't break unexpectedly will help provide some confidence when making changes.