you would need to hand code this on the login success page.
Lets say for exmple sakes that the session that contains the user level is named "UserLevel" and it contains the values "admin" or "visitor"
you could use the following code at line 1 of the login success page:
<?php
if(!session_id()) session_start();
switch($_SESSION['UserLevel']) {
case "admin":
header("Location: admin_index.php");
break;
case "visitor":
header("Location: visitor_index.php");
break;
}
?>