This is a newish issue – that showed up after a recent upgrade.
It seems to highlight CSS just fine – an example from today – http://www.byobwebsite.com/forum/general-thesis-questions/footer-doesnt-sit-at-bottom-of-page/#p25651
But fails entirely with PHP – an other example from today – http://www.byobwebsite.com/forum/general-genesis-questions/problems-showing-correct-image-size-for-portfolio-entries-on-enterprise-pro-homepage/#p25653
I have tried deleting it and redoing it several times, including inserting the code into a blank post, but nothing seems to work.