What is this PHP doing?

1

This is the first lines in the index.php page of phpbb2.

What exactly is the extension.inc doing ?
And these lines from common.
I know they are to stop hacking attempts… But what sort are they stopping.


define('IN_PHPBB', true);
$phpbb_root_path = './';
include($phpbb_root_path . 'extension.inc');
include($phpbb_root_path . 'common.'.$phpEx);



/*
extension.inc
$Id: extension.inc,v 1.5 2002/04/04 11:52:50 psotfx Exp $
*/ 
if ( !defined('IN_PHPBB') )
{
    die("Hacking attempt");
}
// Change this if your extension is not .php!
$phpEx = "php";

$starttime = 0;



// Protect against GLOBALS tricks
if (isset($HTTP_POST_VARS['GLOBALS']) || isset($HTTP_POST_FILES['GLOBALS']) || isset($HTTP_GET_VARS['GLOBALS']) || isset($HTTP_COOKIE_VARS['GLOBALS']))
{
    die("Hacking attempt");
}



// Protect against HTTP_SESSION_VARS tricks
if (isset($HTTP_SESSION_VARS) && !is_array($HTTP_SESSION_VARS))
{
    die("Hacking attempt");
}