If your going to have "regestered users" then its got to be server side. And as you already have experience with PHP/MySQL it looks like the languages your will need are:
- HTML ( You need markup on all sites )
- CSS ( You need this for the styling )
- PHP ( To do the server side security and interactions etc. )
- MySQL ( To hold the data, or atleast hold the usernames/passwords etc. )
- JavaScript ( This is optional, just for user interface really )