error with login

<?php virtual('/Connections/working.php'); ?>
<?php
// *** Validate request to login to this site.
if (!isset($_SESSION)) {
  session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
  $_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['username'])) {
  $loginUsername=$_POST['username'];
  $password=$_POST['password'];
  $MM_fldUserAuthorization = "UsertypeID";
  $MM_redirectLoginSuccess = "/admin/index.php";
  $MM_redirectLoginFailed = "/admin/loginfailed.php";
  $MM_redirecttoReferrer = true;
  mysql_select_db($database_working, $working);
      
  $LoginRS__query=sprintf("SELECT Username, Userpassword, UsertypeID FROM User WHERE Username='%s' AND Userpassword='%s'",
  get_magic_quotes_gpc() ? $loginUsername : addslashes($loginUsername), get_magic_quotes_gpc() ? $password : addslashes($password)); 
   
  $LoginRS = mysql_query($LoginRS__query, $working) or die(mysql_error());
  $loginFoundUser = mysql_num_rows($LoginRS);
  if ($loginFoundUser) {
    
    $loginStrGroup  = mysql_result($LoginRS,0,'UsertypeID');
    
    //declare two session variables and assign them
    $_SESSION['MM_Username'] = $loginUsername;
    $_SESSION['MM_UserGroup'] = $loginStrGroup;          

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