[EvilTerran]

One option would be to ask them to bring in some code they've written, and then talk through it with them in the interview. Even simple stuff like greasemonkey scripts could be pretty informative as to the candidate's level.

I'd be very wary of asking questions about potentially-obscure corners of tools (I don't know how obscure magic methods are in PHP) -- in a real work environment, you could look it up in a moment, but you usually can't in an interview. You could easily trip up a programmer who thrives with the references to hand, but isn't yet confident enough with the tools not to struggle without the references.