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