http://www.sitepoint.com/hhvm-revisited
http://www.sitepoint.com/php-fights-hhvm-zephir-phpng
http://www.sitepoint.com/hhvm-vs-zend-engine-php-6
http://net.tutsplus.com/tutorials/php/hvmc-an-introduction-and-application/
http://www.sitepoint.com/look-hack-php-replacement-hhvm
http://www.sitepoint.com/hhvm-3-1-0-released
http://www.sitepoint.com/hhvm-hack-part-1
http://www.sitepoint.com/easy-wins-performant-php
http://www.sitepoint.com/understanding-opcache/
http://try.appdynamics.com/appdynamics-for-php
http://www.sitepoint.com/horizontal-scaling-php-apps
http://www.sitepoint.com/horizontal-scaling-php-apps-part-2
http://www.webdesignbooth.com/7-areas-of-php-you-might-want-to-optimize/ - 7 Areas Of PHP You Might Want To Optimize
http://net.tutsplus.com/tutorials/php/3-ways-to-speed-up-your-site-with-php/
http://net.tutsplus.com/tutorials/php/oo-php5-adding-caching-to-data-access-layer/
http://www.sitepoint.com/interview-appserver-io-crew/
http://www.sitepoint.com/setting-up-php-behind-nginx-with-fastcgi/
