If you are running a reverse proxy server in front of your webserver, it might sometimes be useful to see which header variables are reaching your webserver. To achieve this, create the following headers.php file in the root of your webserver documents folder:

<?php
echo "<h1>Received Headers</h1>";
$headers =  getallheaders();
foreach($headers as $key=>$val){
  echo $key . ': ' . $val . '<br>';
}
echo "======================"
//echo '<pre>'; print_r($_SERVER); echo '</pre>';

?>