require_once in php

I have a php file which has a require_once Statement (?) this file is then in included in 2 other php files, one php file is in a sub directory so the layout is like this ("file1" and "file2" include the file "included" which require_onces the "required")#

 L--subfolder1
 | L--file1
 L--subfolder2
 | L--requir开发者_Python百科ed
 L--file2
 L--included

How can I reference the "required" file from the "included" file so that it will work from both file1 and file2?

always use absolute path

$_SERVER['DOCUMENT_ROOT']."/included";

or

$_SERVER['DOCUMENT_ROOT']."/subfolder2/required";

would work from anywhere

You have to use absolute path to be able to require the file with the same code from anywhere.

require_once($_SERVER['DOCUMENT_ROOT'] . '/subfolder2/required');

You can use dirname() in combination with the __FILE__ constant. (If I understand you correctly you're including the file 'included' from both scripts and 'included' then includes some files?)
included(.php):

require_once( dirname(__FILE__)."/subfolder2/required" );