Via this post on Digg.com, there's a "PHP Flash Card" application that someone's created using a bit of Ajax to make it all work.

Here's a simple site I created for helping PHP programmers old and new study for certification exams or to simply hone their skills. It has wiki-like features so that users can submit and edit questions and answers.

The site offers a pretty simple interface, but it works that way - too much more and it wouldn't be as attractive. So, head on over and test out your PHP knowledge and maybe even submit some questions of your own...