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I have learned that it's good to approach something like this with a level of humility. What looks like a big pile of spaghetti code may actually have a structure, just not one you may like. I often think "How stupid can these people be?" only to learn later that they actually had a reasonable design. It pays to take the time to understand the code.

Otherwise I'd try to refactor the code into testable modules as much as possible. Unfortunately PHP is not on your side when it comes to refactoring. Especially in the older version people used a lot of "tricks" that make refactoring hard.