Tautan Eksternal
Using doctrine2 for ORM in PHP, http://christoph-burmeister.eu/?p=2268
Doctrine 2 ORM, Object Relational Mapper for PHP 5.3, http://marco-pivetta.com/doctrine2-orm-tutorial/#1
Tree - Nestedset behavior extension for Doctrine 2, https://github.com/Atlantic18/DoctrineExtensions/blob/master/doc/tree.md