title: HTTP_REFERER coming back with NULL, key does not exist in $_SERVER
body: <p>For the first time since using <code>$_SERVER['HTTP_REFERER']</code> it gives me <code>NULL</code> as a result.</p>
  
  <p>When I do <code>var_dump($_SERVER)</code> the <code>HTTP_REFERER</code> key does not exist.</p>
  
  <p>I also try to visit site with different browsers and from different sites but with no result.</p>
  
  <p>The website is running on a Linux/Apache based server.</p>
  
  <p>How do I fix this?</p>