<?php
session_start();
$_SESSION['a'] = "xx";
var_dump(headers_list());
?>


Result 

array
  0 => string 'X-Powered-By: PHP/5.3.8' (length=23)
  1 => string 'Expires: Thu, 19 Nov 1981 08:52:00 GMT' (length=38)
  2 => string 'Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0' (length=77)
  3 => string 'Pragma: no-cache' (length=16)