During my third year I developed a plug-in that applies the MPTT technique to WordPress CMS (it is a GSoC 2009 idea from WordPress – http://codex.WordPress.org/GSoC2009) . I did it as a project for a subject called program project. If we want to add more functionalities to WordPress then it can be done in two ways, implement the code inside the WordPress core or add the implementation as a plug-in. It was a hard thing to deal with WordPress core so I developed it as a plug-in by dividing the project into two parts.
MPTT technique is a great algorithm . . . to get more details refer this site
1. Applying the MPPTT technique to WordPress database – here i added new fields lft and rgt to some of the WordPress database table – wp_post, wp_comment. By rearranging the database i can convert the database into tree structure(MPTT) and this tree structure will be useful in the 2nd step.
2. Showing the contents of WordPress using MPTT – The best way to handle this is to modify the WordPress core but dealing with wordpress core is very hard so i planned to provide a new template that apply the MPTT technique and display the contents of the blog. This new template is not the perfect one but it shows most of the contents(posts and comments) correctly using MPTT technique.
some of the progress documents(RUP) i used to develop the plug-in . . .