Cache control in PHP - and how to test Varnish cache

<?php
//http://stackoverflow.com/questions/4480304/how-to-set-http-headers-for-cache-control


  //set headers to NOT cache a page
  header("Cache-Control: no-cache, must-revalidate"); //HTTP 1.1
  header("Pragma: no-cache"); //HTTP 1.0
  header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // Date in the past

  //or, if you DO want a file to cache, use:
  header("Cache-Control: max-age=2592000"); //30days (60sec * 60min * 24hours * 30days)

  //Check Varnish cache
    //http://www.isvarnishworking.com/
?>