title: use of $_SERVER['HTTP_REFERER']
body: <p>In logout page I have seen use of</p>
  
  <pre><code>       $http_referer=$_SERVER['HTTP_REFERER'];
         header('Location :'.$http_referer);
  </code></pre>
  
  <p>Where does this header redirecting page to and for what http_referrer is used for</p>