Simple script for viewing the same site from different URLS

This php script enables working on the same website without wondering about the location from which you view it.
For example, you have the website files in a folder of your local webserver, and when you put it online, you must access it from another host name.
Examples:
http://localhost/mywebsite/index.php
http://192.168.0.100/mywebsite/index.php (accessing your local machine from another computer on the same network)
http://www.mywebsite.org/index.php (your website on the remote webserver).

This script can be placed everywhere after it'd properly configured (the folder in which the page is must be set).

//common: folder containing this file relatively to the website root
$conflen=strlen('common'); 
$B=substr(__FILE__,0,strrpos(__FILE__,'/')); 
$A=substr($_SERVER['DOCUMENT_ROOT'],strrpos($_SERVER['DOCUMENT_ROOT'],$_SERVER['PHP_SELF']));
$C=substr($B,strlen($A));
$posconf=strlen($C)-$conflen-1;
$D=substr($C,1,$posconf); 
$host='http://'.$_SERVER['SERVER_NAME'].'/'.$D;
?>

Note: this works bot on IIS based webservers and Unix based.