Simon Holywell has posted what he calls a "showdown" between HHVM, Zephir and PHP comparing various benchmarks (based on a Mandelbrot Set fractal).

Since its inception the slow running speed of PHP has been widely publicised and over the years there have been a number of improvements. [...] It has become more interesting recently however with three projects looking for improvements in different ways. The core has adopted the Zend OPcache for future versions of PHP, Facebook has been working on a just in time compiler called HipHop VM and the team that brought us Phalcon framework have created Zephir.

All of these projects have chosen to tackle the issue of PHP's speed via different avenues. It has therefore left one simple question - who's making the biggest improvements? Who's the fastest?

He briefly introduces the "contenders" for those not familiar with them and gets right into the benchmarking process. He shares the link to the tests he used and a few notes about the HHVM setup that could account for lower numbers. He shares his results in a few graphs or you can grab the CSV data yourself and parse it. The entire setup is also over on GitHub if you'd like to just check that out.