<?# - server_var.php
echo $_SERVER['HTTP_USER_AGENT'];
?>
CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
If Client's browser is MSIE, or Firefox, then the related term is included into the string that is returned from this server variable.
... to search for any typical defined part in this variable we can use the function strpos(); as follows :
<?
if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== FALSE)
{echo 'You are using Internet Explorer.<br />';}
if(strpos($_SERVER['HTTP_USER_AGENT'], 'Firefox') !== FALSE)
{echo 'You are using Firefox.<br />';}
?> . . .
So the server recognizes through the above script if you are using MSIE or Firefox Browser.
If none of the 2 browsers are requesting this page to the server, then this script returns nothing.