Best way to get started with web development, especially php? Going to maintain an enterprise application without having any web development experience.

Hello world, to do app, blog. Just try to develop stuff like those. Needs your hands dirty. Once you feel a bit comfortable, dig into that enterprise application source code. If you don't have access yet, look for similar/alternative open source solutions out there so you can see how things work.

Get to know the business before you get to know the code base.
Learn how the business works as the software usually works in a similar way to automate their processes.

A good read is:
https://www.phptherightway.com/

If they are using unit testing it’s most likely phpunit.
https://phpunit.de/

Expect a chaotic mess 😏this way if it is, your expectations are already low enough to handle it 🙃