Added new internals article.

README.md

@@ -747,6 +747,7 @@ Various resources, such as books, websites and articles, for improving your PHP
 * [How Foreach Works](http://stackoverflow.com/questions/10057671/how-foreach-actually-works/14854568#14854568) - A detailed StackOverflow answer about foreach.
 * [When Does Foreach Copy?](http://nikic.github.com/2011/11/11/PHP-Internals-When-does-foreach-copy.html) - An article about the internals of foreach.
 * [How Big Are PHP Arrays (And Values) Really?](http://nikic.github.com/2011/12/12/How-big-are-PHP-arrays-really-Hint-BIG.html) - An article about array internals.
+* [Why Objects (Usually) Use Less Memory Than Arrays](https://gist.github.com/nikic/5015323) - An article about object and array internals.
 * [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.