2015 UPDATE: new wiki page for HHVM on CentOS7 with alternate instructions
While building HHVM (HipHop Virtual Machine) for faster PHP on CentOS 6.x was pretty much a nightmare because almost none of third-party libraries are available from major repositories, the freshly released CentOS 7.0 solves most of this problem as it has far newer packages available from EPEL (because it has roots in more modern Fedora 19).
Note that unlike PHP/PHPNG, HHVM requires major resources to build. You may be used to making PHP in 15 minutes on a little old 256mb 1ghz box. You can forget that with HHVM which is a beast to compile. 1gb ram minimum and lots of cpu power required to keep it an hour or two (this is part of why PHP NG instead is so exciting because it will be so much easier to custom build).
Zeev Suraski (CTO of Zend and therefore an extremely qualified authority on PHP NG) has written a fascinating article comparing the performance of PHPNG against the current HHVM versions. A must read.
Perhaps even more importantly, he has written about the current state and future of PHP NG.
correction: php core developers have urged that it is improper to call this version “5.7” (despite the versioning file stating so)
PHP 5.7 PHP NG is still in alpha development, however it is starting to show breathtaking performance improvements over 5.6 while maintaining virtually complete compatibility.
Dmitry Stogov has been hard at work since his first announcement in mid-January 2014 and milestone update in early-May to keep folding in more and more ideas to increase PHP speed (with significant contributions by Xinchen Hui, Nikita Popov and others).
Six months later in mid-July, their efforts are really bearing fruit and PHP
5.7 NG is about to become nearly 100% faster than PHP 5.6 when rendering the front page of a stock WordPress 3.6 installation:
PHP 5.6, 1000 renderings of WP front page = 26.756 seconds
PHP NG, 1000 renderings of WP front page = 14.810 seconds