You should make sure to set expectations. If your employer only wants the minimal amount of maintenance done, then don't do any more. You could go to heroic lengths to repair the codebase, but if that's not what they're asking for it will be in vain.
Second, I suggest applying as many tools as you can. A modern version control system, of course, and keeping any version control history that you inherited (although it sounds unlikely).
A powerful IDE might also let you start cutting out crap immediately, so try PHPStorm or Eclipse+PHP (or both!) and see what they can tell you.
And start writing tests as you start making changes, because you'll likely break something seemingly unrelated when you start changing things.
Comments
You should make sure to set expectations. If your employer only wants the minimal amount of maintenance done, then don't do any more. You could go to heroic lengths to repair the codebase, but if that's not what they're asking for it will be in vain.
Second, I suggest applying as many tools as you can. A modern version control system, of course, and keeping any version control history that you inherited (although it sounds unlikely).
A powerful IDE might also let you start cutting out crap immediately, so try PHPStorm or Eclipse+PHP (or both!) and see what they can tell you.
And start writing tests as you start making changes, because you'll likely break something seemingly unrelated when you start changing things.