PHP Session problem

<?php

header('Content-Type: text/html; charset=UTF-8');

$main_domain = $_SERVER["HTTP_HOST"];
$expld = explode('.', $main_domain);

if(count($expld) > 2) {
   $tld = array_pop($expld);
   $domain = array_pop($expld);
   $main_domain = $domain . "." . $tld;
}

$main_domain = ".localhost";

session_name('sid');
session_set_cookie_params (0, '/', $main_domain);
session_start();
echo session_id();
exit;


?>