using the login and authentication wizard through Dreamweaver 8

here is the code I am using..
<?php require_once('Connections/COnn2.php'); ?>
<?php
// *** Validate request to login to this site.
//////////when I delete this---if (!isset($_SESSION)) {
session_start();}-----it gets rid of the error messages but when I logon I go the registration page either and not the logon page,
/////////
$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
$_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['firstname'])) {
$loginUsername=$_POST['firstname'];
$password=$_POST['lastname'];
$MM_fldUserAuthorization = "";
$MM_redirectLoginSuccess = "User_login.html";
$MM_redirectLoginFailed = "registration.html";
$MM_redirecttoReferrer = false;
mysql_select_db($database_COnn2, $COnn2);

$LoginRS__query=sprintf("SELECT id, f_name FROM auth_users WHERE id='%s' AND f_name='%s'",
get_magic_quotes_gpc() ? $loginUsername : addslashes($loginUsername), get_magic_quotes_gpc() ? $password : addslashes($password));

$LoginRS = mysql_query($LoginRS__query, $COnn2) or die(mysql_error());
$loginFoundUser = mysql_num_rows($LoginRS);
if ($loginFoundUser) {
$loginStrGroup = "";

//declare two session variables and assign them
$_SESSION['MM_Username'] = $loginUsername;
$_SESSION['MM_UserGroup'] = $loginStrGroup;

if (isset($_SESSION['PrevUrl']) && false) {
$MM_redirectLoginSuccess = $_SESSION['PrevUrl'];
}
header("Location: " . $MM_redirectLoginSuccess );
}
else {
header("Location: ". $MM_redirectLoginFailed );
}
}
?>


any suggestions??