what i did was that i used the same session variable for username whether the user is an admin or member, then created a seperate file ie: isadmin.php and included that in all the pages where the admins had privilages...
inside the isadmin.php is something along the lines of:
include("dbconnectdetailia.php");
if (!empty($_SESSION['username'])) {//the session variable username is not empty so the user is logged in
$isadmin = mysql_query("SELECT admin FROM members WHERE username='".$_SESSION['username']."' ") or die(mysql_error());
$isadmin = mysql_fetch_array($isadmin);
$isadmin = $isadmin['admin'];
}
//if $isadmin = 1 user is an admin, if $isadmin = 0 user is not an admin
N.B.
In my members table i had a field which is either a 1 or a 0 depending on whether the user has admin privilages.