PHP login auth

<?php
//File will check if user is logged in.
//If user is not logged in, will be redirected to the login page with an error.
//Logged in users will be allowed access to the page.
//Copyright 2012 Cas Eliëns and Dennis Koeter.
$q=$_GET['q'];
//$a is authentication level required.
//if $a is 'u', regular user authentication is required.
//if $a is 'a', administrative authentication is required.


if($q=='u') {
    if(isset($_SESSION['uid'])) {
        $uid = $_SESSION['uid'];
        }
        else {
        header("location:index.php?q=noauth"); }
    }

elseif($q=='a') {
    if(isset($_SESSION['perm'])) {
        $perm = $_SESSION['perm'];
        $uid = $_SESSION['uid'];
        if($perm=='a') {
            $authenticate = true;
            }
        elseif($perm=='u') {
            header("location:noauth.php?q=u");
            }
        else {
            header("location:noauth.php?q=e");
            }
        }
    }

else {
    header("location:index.php?q=noauth");
    }
?>