PHPPHP is well on its way to being a simple PHP command line debugger. To my knowledge there currently isn't a good tool for command line debugging on PHP.
It seems like a case of sensibility, rather than irony, to me. Using Python, even when developing PHP-related software, just seems like the intelligent thing to do.
xdebug exposes a gdbp interface which supports breakpoints, stepping, inspecting variables, etc. And you can use it with gdb or any GUI that supports gdbp, such as macgdbp.
EDIT: It does not expose a REPL though, which I suppose is what you meant?
