I have been learning php for sometime now. Besides reading books and doing tutorials i also like to sometimes read other programmers code. I have came across this:

<?php

    // authentication. 
    include_once($sys_root."/inc/authenticate_check.php");

I understand the use for $sys_root, but what i dont understand is, how is $sys_root bieng assigned a value.

I have checked and made sure that $sys_root is not a special var of PHP's.