This might sound sacreligious to the many vim fans here, but get a good IDE, it will help you get a handle on what the code is doing and let you navigate around

This might sound sacreligious to the many vim fans here, but get a good IDE, it will help you get a handle on what the code is doing and let you navigate around faster, which is especially handy if the execution path for accomplishing any one thing involves dozens of files. A good IDE will also point out blatant errors, and a really good IDE will point out potential errors as well. I personally really like PHPStorm by JetBrains, the code inspection tool is quite good. I was recently able to cut the size of our code base in half by using it to identify tens of thousands of bugs, a lot of them on inspection were "this never worked" type bugs, which with a little digging I was able to confirm could never be called. Eliminating code also makes refactoring the remaining code easier because you have fewer interdependencies to worry about.