small website in php

To build a small website that is password protected requiring users to log on and post and retreive information The postings will be specific ideally using a form and then making the information entered dynamically available on the website. Each user will have seperate log in but everyone will see the same information, categorised by user Example User A enters information on the form in relation to user B. That information is then live on the website for all users to see. The posting will be categorised under user B As new users are added the existing users should be able to post information on the new user. The site is a trade site to share information.