Because when a user login to your website you need to authenticate them, which means you need a authentication system to log them in.
If it's an admin login then you redirect them to your admin dashboard.
In controllers views etc; Just create an admin folder in each for the admin files.
What did you Try? What did you Get? What did you Expect?
Joined CodeIgniter Community 2009. ( Skype: insitfx )
