Apparently PHP is capable of being interactively debugged in an IDE, using only FOSS software. I'm still researching this. Apparently there are a number of front-ends (IDEs) which support the necessary protocol, including KDevelop, and there's at least one plug-in on the back-end.
It may be necessary to run PHP and the IDE on the same machine.
- phpdbg - function library. There is a page for each function, but not a lot of examples or overall explanation of how to make it work.
- XDebug: Debugger and Profiler Tool for PHP
- /VSC: using Visual Studio Code
Language Server Protocol (LSP)
It's not clear whether LSP is any help with interactive debugging; it seems primarily aimed at editing.
- langserver.org is "A community-driven source of knowledge for Language Server Protocol implementations", including several for PHP.