When facing legacy code, probably the first thing you want to do is clean it up. PhpStorm 2019.1 can help you with this, particularly by finding and removing dead code with the new Unused Declaration inspection. It will carefully analyze your code and data flow, and highlight the entities that are never used.
To get started, make sure that the inspection is enabled under Settings/Preferences | Editor | Inspections | PHP | Unused. Now, when you work in the editor, PhpStorm will only highlight unused classes, class members, and functions:
To locate and view all unreachable declarations