Question

When the Customer or Admin have logged in, their member_id will be stored in the session ($_SESSION['member_id'])

There are number of group levels, is it better to store in the session like this:

$_SESSION['group_id'] = 2;

or check the group level from the database?

SELECT group_id FROM users where member_id = .  $_SESSION['member_id'];

I have written Users Login class for the frontend, there is nothing wrong using same class for the backend?

Answer 1

If you have the member_id stored in a session variable, why not have group_id ? session variables allow persistance in your data between requests and that seems to be what you need.

Although some do recommend to keep session data at a minimin, ie:

 $_SESSION['uid'] = 5;
 $_SESSION['logged_on'] = true;

I would not consider having group_id as bad usage of session vars since in your case member_id depends on group_id to do anything.