Ariz Jacinto has pointed out some results from a recent benchmarking "game" that compares the performance of several languages against each other in overall speed.
Here's the link to the Computer Language Benchmark Game as of July 15, 2012. You'll see that the median speed of PHP is slightly faster than Python 3 and Ruby 1.9 on x64 Ubuntu/IntelQ6600 Quad-core machine.
You can visit the results page and turn on and off various languages to get the updated results in the chart. You can also limit it down to one of the many benchmarking test types as well.