I finally got my "dynamic PHP code optimizer" idea to yield measurable results!! Yippeee! I'm not going to talk too much about it because it'll be in my O'Reilly presentation, but it's really cool. Basically, it loads up large include files in memory without passing it through the PHP engine, then dynamically loads individual functions as they become necessary. In IlohaMail's main.php, I'm getting a 7-9ms improvement (which is about 5-8% of the overall execution time), although it's highly dependent on the script and usage.