There are some useful link which help to make a nested tree program.
http://dev.mysql.com/tech-resources/articles/hierarchical-data.html
http://iamcam.wordpress.com/2006/03/17/storing-hierarchical-data-in-a-database-part-1
http://www.devnetwork.net/forums/viewtopic.php?p=306991&sid=69bad305db7a95180f600b7983c8e813
we also used a php class tree(db_mysql.class.php, dbtree.class.php, dbtree_visual_demo.php, dbtree_demo.php).