problem with php restrict access to page()

Hi, i'm using dw cs5 for my website and using php for updating info in the database. However, I have a problem with the restict access to page() behaviour.

I have a login page which either takes me to the update info page or will take me to the "restricted" page. The problem is even with a successful login, I am taken to the 'restricted' page.

I shall paste the code in one of the update pages. Is there anything wrong in it?

<?php
if (!isset($_SESSION)) {
  session_start();
}
$MM_authorizedUsers = "";
$MM_donotCheckaccess = "true";

// *** Restrict Access To Page: Grant or deny access to this page
function isAuthorized($strUsers, $strGroups, $UserName, $UserGroup) {
  // For security, start by assuming the visitor is NOT authorized.
  $isValid = False;

  // When a visitor has logged into this site, the Session variable MM_Username set equal to their username.
  // Therefore, we know that a user is NOT logged in if that Session variable is blank.
  if (!empty($UserName)) {
    // Besides being logged in, you may restrict access to only certain users based on an ID established when they login.
    // Parse the strings into arrays.
    $arrUsers = Explode(",", $strUsers);
    $arrGroups = Explode(",", $strGroups);
    if (in_array($UserName, $arrUsers)) {
      $isValid = true;
    }
    // Or, you may restrict access to only certain users based on their username.
    if (in_array($UserGroup, $arrGroups)) {
      $isValid = true;
    }
    if (($strUsers == "") && true) {
      $isValid = true;
    }
  }
  return $isValid;
}

$MM_restrictGoTo = "login.php";
if (!((isset($_SESSION['MM_Username'])) && (isAuthorized("",$MM_authorizedUsers, $_SESSION['MM_Username'], $_SESSION['MM_UserGroup'])))) {  
  $MM_qsChar = "?";
  $MM_referrer = $_SERVER['PHP_SELF'];
  if (strpos($MM_restrictGoTo, "?")) $MM_qsChar = "&";
  if (isset($_SERVER['QUERY_STRING']) && strlen($_SERVER['QUERY_STRING']) > 0)
  $MM_referrer .= "?" . $_SERVER['QUERY_STRING'];
  $MM_restrictGoTo = $MM_restrictGoTo. $MM_qsChar . "accesscheck=" . urlencode($MM_referrer);
  header("Location: ". $MM_restrictGoTo);
  exit;
}
?>