PHP $_SESSION Error - Headers already sent [duplicate] OR White In PHP Server

You can't start a session after you've sent output to the browser.. Put the <html> tag above the <head> tag.

<?php 
require_once ('config/config.php');
require_once ('config/db.php');
require_once ('classes/Login.php');
$login = new Login(); ?>
<html>
<head>

php