Added PHP Internals Book

README.md

 * [PHP Evaluation Order](https://gist.github.com/nikic/6699370) - An article about evaluation order in PHP.
 * PHP Source Code for Developers: [1](http://blog.ircmaxell.com/2012/03/phps-source-code-for-php-developers.html) [2](http://nikic.github.com/2012/03/16/Understanding-PHPs-internal-function-definitions.html) [3](http://blog.ircmaxell.com/2012/03/phps-source-code-for-php-developers_21.html) [4](http://nikic.github.com/2012/03/28/Understanding-PHPs-internal-array-implementation.html) - A series about the PHP source code.
 * Collecting Garbage: [1](http://www.php.net/manual/en/features.gc.refcounting-basics.php) [2](http://www.php.net/manual/en/features.gc.collecting-cycles.php) [3](http://www.php.net/manual/en/features.gc.performance-considerations.php) - A series about the PHP garbage collection internals.
+* [PHP Internals Book](http://www.phpinternalsbook.com) - An online book about PHP internals, written by 3 core developers.
 
 # Contributing
 Please see [CONTRIBUTING](https://github.com/ziadoz/awesome-php/blob/master/CONTRIBUTING.md) for details.