Warning: mysqli_connect(): (HY000/1045): Access denied for user \'username\'@\'localhost\' (using password: YES) in C:\\Users\\xampp\\htdocs\\PHP_Login_Script
$db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);
Put " DB_HOST " instead of " DB_SERVER " also
put " DB_USER " Instead of " DB_USERNAME "