Why can't a file be included from a folder?

Question

PHP

The site worked on hosting. On home computers, on win10 64, Denwer, everything worked.
Sent to the customer on his win10 (sent both the website and Denver in the archive (downloaded the latest version - www.denwer.ru/dis/Base/127001/Denwer3_Base_2013-06... ) and not a single file is included in the folder:
Z:\home \localhost\www\php
The site is very simple: Index.php and in it, at the beginning:
<?php
require_once "/php/log.php"; Removed the
leading slash and it worked!
same?

Answer 1

require_once "/php/log.php";you should have this folder with the file in the root of drive Z:
It's better to do this:

define('__ROOT__', dirname(dirname(__FILE__)));
require_once(__ROOT__.'/php/log.php');