From LearnComputer.com there's a few hints they've put together that might help you with that next PHP-related job interview.

So you’ve been slinging resumes for a while and now you have an interview for an awesome PHP job. While part of the interview will be the typical job interview, you should also be prepared for a technical interview. Technical interviews are often given to determine how well you truly know the technologies with which you’ll be working. There are numerous books and articles to help you prepare for the job interview portion but very little has been said on preparing for a PHP technical interview.

They've broken down their advice into a few different sections including some sample PHP questions you might get asked about both PHP and MySQL, a quick look at frameworks and the recommendation of the book PHP and MySQL Web Development to help round out your knowledge.