Lorna Mitchell has put together a set of benchmarks for PHP 5.6 comparing them to the three previous minor versions, PHP 5.5, 5.4 and 5.3 based around the same setup as her previous benchmarks of PHP 5.4.
A while ago I did some benchmarks on how different versions of PHP perform in comparison to one another. This isn't a performance measure in absolute terms, this was just benchmarking them all on the same laptop while it wasn't doing anything else, and averaging the time it took to run the benchmark script. Recently I ran it again for versions PHP 5.3 through PHP 5.6 and I thought I'd share my results.
There's a steady drop in execution time over the series of versions, with PHP 5.6 coming in the shortest. She also includes the actual numbers of the results in case you'd like to chart them out yourself.