Hello!
I know perfectly how to develop the site you need, as I have wide experience in working with different bitcoin clients. As you said you are not familiar with bitcoin I will explain a few things to you:
In order to your website works with bitcoins, your server will need to have a bitcoin client installed. There is a command-line bitcoin client which is perfect for this task, and it has a PHP api, so I would build it using PHP. Coding notation will be done.
Regarding to the requirements of your server in order to have a bitcoin client installed, I must say you will need a server with at least 50 GB of free disk space, as bitcoin's blockchain will be completely downloaded by the client.
Another required feature is the ability to control your server with its terminal (via remote connection using PuTTY). If your server would run Ubuntu 12.04 it would be perfect, but this is not required (but very good!).
Regarding to security, I have it covered. You can run all the vulnerability test you want and you won't be able to stole even a satoshi (0.00000001 bitcoins).
Regarding to the login/logout feature, I can develop it as you want with no problem.
If you feel interested in my bid, please contact me as I am interested in this project. I can give you more details. However, I won't be able to start working in this project until 6-7 days from now, as until then I will be busy working in other projects, inside and outside freelancer.com.
Good luck and have a good day! :)