Jacob Pitassi and I presented at DrupalCamp LA to provide some very basic PHP knowledge that will make for better themers. Jacob does a lot of the technical interviewing for Stauffer and has found that many people claim to be themers, but do not know a lick of PHP. While he is not expecting themers to write complicated functions, he does expect them to know how to pull an element out of an array to display.
This is the video of our presentation and, as intended, provides some basic PHP Knowledge. It also explains the use of preprocess function to help out with improving the flexibility of your theme. If you have any questions feel free to contact us or comment and we will do our best to help.