Move Interals Book up list.

README.md

 *Reading materials related to the PHP internals or performance.*
 
 * [PHP RFCs](https://wiki.php.net/rfc) - The home of PHP RFCs (Request for Comments).
+* [PHP Internals Book](http://www.phpinternalsbook.com) - An online book about PHP internals, written by three core developers.
 * [Print vs Echo, Which One is Faster?](http://fabien.potencier.org/article/8/print-vs-echo-which-one-is-faster) - An article about print and echo performance.
 * [The PHP Ternary Operator. Fast or Not?](http://fabien.potencier.org/article/48/the-php-ternary-operator-fast-or-not) - An article ternary performance.
 * [Disproving the Single Quotes Myth](http://nikic.github.com/2012/01/09/Disproving-the-Single-Quotes-Performance-Myth.html) - An article about performance of single and double quoted strings.
 * [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.