Learn to use the debugger and read code commit history. And talk with your team-mates. In the last few months, as a student working part-time (< 20hrs/week), I am on a PHP codebase with over 5 million LOC and 15 years of work, and I was pretty good with dealing around it within a month. There are lots of dependencies like you mentioned and at least three different "eras" of code frameworks and hardly any documentation. I had not touched PHP in any capacity before this for at least 3 years. I admit PHP is easier to read and reason than Java when afresh but those three things made me really fast to be productive and making independent production-ready features.