The TopTal.com site has posted some suggestions on things to ask when interviewing PHP developers, especially those shooting for a senior level role.

Ubiquitous…that is definitely one word you could use to describe PHP in relation to the web. It really is everywhere. [...] What makes PHP so popular and widely-used? While there’s no single answer to this question, PHP’s ease of use is certainly a significant contributing factor. [...] But therein lies much of the challenge of finding highly-skilled PHP developers. PHP’s relatively low barrier-to-entry and 20 year history means that PHP developers have become practically as ubiquitous as the technology itself. Yet while many can legitimately claim to “know” PHP, those who are true experts in the language are capable of producing software that is much more scalable, functional, robust, and maintainable.

There's a wide range of questions included in their list, each one with a brief description and the "right" answers a knowledgable candidate might give. This includes questions about:

• Defining and using closures
• What "global" is and when to use it
• Describing the PHP superglobals
• The use of "static"

There's also a section for the even more advanced development positions out there with questions about PHP's internals (the actual C code) as well as the differences between some built-in object types.