title: in what situation does the HTTP_REFERER not work?
body: <p>I have used referrer before in foo.php to decide whether the page iframing foo.php is of a particular URL.  (using <code>$_SERVER['HTTP_REFERER']</code>)</p>
  
  <p>It turned out that most of the time, it worked (about 98% of the time), but it also seemed like some users arrived the page and <code>$_SERVER['HTTP_REFERER']</code> was not set in foo.php and therefore broke the code.  [<strong>update</strong>: These user claimed that they followed the usual page flow and didn't use the URL of foo.php all by itself on the browser (that they let it be an iframe) and the users never altered their browser settings.]</p>
  
  <p>I wonder what the reasons are that it could happen?</p>