Notice: Undefined index: HTTP_REFERER in C:\xampp\htdocs\cart\order.php on line

<?php

define('INCLUDE_CHECK',1);
require "connect.php";

if(!$_POST)
{
    if($_SERVER['HTTP_REFERER'])
    header('Location : '.$_SERVER['HTTP_REFERER']);

    exit;
}

?>

You should disable notice.
Add below line on top of your php page.

error_reporting(0);

You can also disable warnings globally by changing in php.ini file.