In a recent blog post Zoe Slattery took a look at opcodes in PHP, specifically related to performance increases.
After one particularly frustrating day [looking at how the Zend engine works] I asked a colleague (Andy Wharmby) about the issue I was looking at. Then a miracle happened - he'd not only looked at it but had documented it (and many other things) in a set of charts.
She has linked to the charts (Open Office format) for the results of some tests her friend had already run. They're a bit out of date and some parts might be inaccurate, but they're still a good start.