Recently I saw a new database management system that was coded in php. The concept was first implemented in vb6 and was a high rated submission PSC-VB world. McDataBase is a new, high speed, open source database management system which is very easy to use and highly customizable. Actually, it is file based database system which uses different files for each tables.

You can download its sample code by clicking here
There is a sample comment system made using this database system download it