On the Blackfire.io blog Julien Pauli peeks behind the curtains of PHP. In the five part series he explains how you should write your code to make the best use of the internal optimizations present in PHP 7.

This blog series will show you what changed inside the Zend engine between PHP 5 and PHP 7 and will detail how you, as a developer, may effectively use the new internal optimizations. We are taking PHP 5.6 as a comparison basis. Often, it is just a matter of how things are written and presented to the engine. Performance must be taken care of when critical code is written. By changing some little things, you can make the engine perform much faster, often without losing other aspects such as readability or debugging control.

https://blog.blackfire.io/php-7-performance-improvements-packed-arrays.html