As part of a project he was working up, Cory Rauch has created some statistics comparing the performance of the latest PHP 5 series release, PHP 5.2, with the previous version, PHP 5.1.
I can't say the results of the benchmarks are surprising, but it does give a good idea of where, speed-wise, they really improved things in this new release. There are some stats, though, that didn't make that much of a jump. Those seem to be ones dealing with objects and their handling, though - so there's not that much of a jump there anyway.
He tested all sorts of operations, including working with binary data, string and array functionality, looping (for, foreach, etc), and operators. For each statistic, he provides what the test is doing and what the results are. Testing was done with the PHPLens benchmark suite.