Header Redirct with vairables not working

I have a log in form, once filled out if successfully logged in, it should go back to the same page url including the url variables. It works, but does not carry the url varables (?barnid=123) with it.

<?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 = "";
  $MM_redirectLoginSuccess = $_SERVER['PHP_SELF'].'?'.$_SERVER['QUERY_STRING'];
  $MM_redirectLoginFailed = "login_fail.php";
  $MM_redirecttoReferrer = false;
  mysql_select_db($database_admin, $admin);
  
  $LoginRS__query=sprintf("SELECT username, password FROM users WHERE username=%s AND password=%s",
    GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text")); 
   
  $LoginRS = mysql_query($LoginRS__query, $admin) 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['PrevUrlx']) && false) {
      $XXMM_redirectLoginSuccessXX = $_SESSION['PrevUrl'];	
    }
    header("Location: " . $MM_redirectLoginSuccess );
  }
  else {
    header("Location: ". $MM_redirectLoginFailed );
  }
}
?>

What could i be doing wrong?

Start by changing header to echo and make sure the value is what it should be.

If the from user submits his username and password is the same page with php code,

provide $_SERVER[QUERY_STRING] in action attribute of form.

<form method="post" action="<?php echo $_SERVER[php_SELF].'?'.$_SERVER[QUERY_STRING]; ?>">