Wikipedia on HHVM
They note that 5.6 has some substantial performance improvements over 5.3. I'd point out that PHP 7, coming out later this year, will be even faster (perhaps as much as 2x 5.6), since some very smart people have done some big internal refactoring in the Zend engine, largely to reduce allocations, pointer indirection, and memory usage. These efforts combined have resulted in pretty big performance gains, making PHP 7 more competitive with HHVM performance-wise. In addition, writing PHP extensions is somewhat nicer now.<p>Zeev Suraski (one of the original two Zend Engine authors along with Andi Gutmans, also co-founder of Zend, Inc.) did some benchmarks:<p><a href="http://zsuraski.blogspot.co.uk/2014/07/benchmarking-phpng.html" rel="nofollow">http://zsuraski.blogspot.co.uk/2014/07/benchmarking-phpng.ht...</a><p><a href="http://zsuraski.blogspot.co.uk/2014/07/benchmarking-phpng-magento.html" rel="nofollow">http://zsuraski.blogspot.co.uk/2014/07/benchmarking-phpng-ma...</a><p>However, bear in mind that those benchmarks were faked* by Zeev, a Zend Engine proponent. Benchmarks faked* by HHVM engineers might show a larger gap. ;)<p>* I'm not using "faked" in its normal sense. They probably weren't fraudulent, but I am a believer that we should say "fake a benchmark" and not "make a benchmark" given that they can be so circumstantial and misleading: <a href="http://blog.ajf.me/2014-10-23-on-benchmarks" rel="nofollow">http://blog.ajf.me/2014-10-23-on-benchmarks</a><p>Disclaimer: I am, myself, biased towards the Zend Engine, I'm a php-src committer.