Retrieve information from different sites

I have several sites on one server and have a script to show users online and it requires me to install a snippet on the sites index page it then writes the information to a txt file how could I make it so the file that has the line

<?php
include('visitor.php'); 
?>

PHP:

Would be able to retrieve this same information from all the domains on the server?

I have tried to put the actual address to the file and it didnt work

 

Edit your php.ini file to add a prepend file, or add that snippet to all files on all domains (making sure the visitor.php and include()'d files exist).

Jay