Sounds like an OK test to me. Great (senior) developers should be able to do that kind of thing. Categorizing yourself exclusively as "a Ruby developer" is a career trap.
Know what I'd do if the interviewer asked me to debug PHP? Pretty much return the question:
"I've never used PHP. Are there logging macros/functions defined somewhere? Where do I see the output? syslog maybe? Is there a debugger of some sort I can use? How do I run each `piece` of code in isolation?"
(I am assuming the job listing did not explicitly mention PHP experience. If it did, both myself and the recruiter would absolutely deserve to fail me for this interview).