Best way to debug my way through a newly acquainted large PHP app and detect dead code?
As per the discussion in the comments, I'd wondered if the execution coverage tools in XDebug would help here. This is a low-level code monitor that watches what lines of PHP code have been executed when you start a program. As you'd expect, this adds a certain level of overhead, so it is generally not added to live environments.
Hosted build systems like Travis and Scrutinizer do code coverage reporting, though they probably rely on tools like PHPUnit and XDebug to actually make them work.