Piecing this together from against other answers it sounds like you need multiple application streams.
That is, you have a situation where you need multiple "users" to be logged in to the application on different tabs on the same browser, same machine.
This isn't because they are different people using the machine, but rather the same person working with different personas.
It turns out, I've implemented something similar in the past myself, in order for managers to be able to "ghost" through a system as their staff members. They log in as the other user, but in a read only mode so they can see what's going on.
OK. So how to do it.
Put simply - the session isn't enough - you need more than that. The session ID is stored in a cookie on the client machine and there isn't really much you can do about the set-up - one browser = one session.
However, what you can do is split that session up with an application stream, or application context.
That is, don't store anything in the root of your session - split your session into distinct components into which you have a set-up identical to your current session.
The key for each session is then the "application stream" key. You need to pass this around in your URLs.
E.g.
Your current session may have a simple set-up:
$_SESSION['user'] = 'some username';
$_SESSION['role'] = 'power user';
Instead you store that as:
$_SESSION[0]['user'] = 'some username';
$_SESSION[0]['role'] = 'power user';
On all urls you add:
&appId=0
And whenever you reference your session you use something like:
$username = $_SESSION[ $_GET['appId'] ]['user'];
Obviously, you wrap all this up in a nice session handling class, but that's the basic idea.
If you want a link that generates a new login page with a new application stream, you simply change the appId on the link (or completely omit it and trap that in your login code).
E.g.
$sLoginLink = "<a href='/login.php?appId=" . generateNewAppStreamId() . "' target='_BLANK'>New Login Screen</a>";
As everything is still stored in the session, the whole of your application should work exactly the same - just as long as you always have the appId on every URL in the system.
I've tried to make the explanation as simple as possible - forgive me if I've used too many words.
