php-src/ext/spl

This is an extension that aims to implement some efficient data access 
interfaces and classes. You'll find the classes documented using php
code in the file spl.php or in the corresponding .inc file in the examples
subdirectory. Based on the internal implementations or the files in the 
examples subdirectory there are also some .php files to experiment with.

For more information look at: http://php.net/~helly/php/ext/spl