User Level Access Control Priviledges

Hi all, I am encountered with what seems a potentially difficult system to implement. I need to implement a user level access control system. The application I will create is a consultancy application, with various user categories, where each user category will have access rights to a particular table (or possibly individual fields in each table). A user group can have the following: no access, read access, write access. and depending on their group, they will be able to view and manipulate certain pieces of information that are contained in tables. What I want, is more or less similar to the way in which MySQL implements privileges and level control for users. I originally planned to implement this into MySQL database, but now feel that it would be best if I implemented were to code it. Yet, I may still implement it in MySQL if its a simple enough solution. I am to look into both methods. Does anyone know how I can best implement this (or would be able to start me off so that I could take it on further), or know of any online tutorials? Your help is greatly appreciated. Tryst