How to correctly load a PHP file in another PHP file?

I have trouble to find a way to load a php file correctly in another php file when the php file is included in another php file with include () and required_once(). I found that if Afile.php uses url such as ../controller/mycontroller.php to include another php file in it, when Afile.php in been included in Bfile.php which is located in a different dir, the ../ will lead the Afile.php's url to a different dir that is based on Bfile.php.

I tried to use these PHP string to specify a file's url with a static one, so that there is no misunderstanding when there is multiple include among many .php files. But it did not work!

Is there a better way to define a file location of a .php file so that when I want to include, call or load the .php file that is in a different dir while itself is also included in other php file in other dir?

If there is any confusion please let me know.

Thank You

Update

define('PUBLICPATH', __DIR__.'/');
// I prefer to have the "root dir" not accessible from public dir
define('ROOTROOT', realpath(PUBLICPATH.'..').'/');
$pathnow= define('APPPATH', realpath(PUBLICPATH.'../application').'/');
echo 'pathnow:'.APPPATH;

It returned pathnow:/ as result page in chrome

Solution

The easiest thing is to define one constant ROOT_PATH (or more) in the first called script, then to use it in every include/require instruction. For example let's say you have the following tree:

/var/www/example.com            - 
/var/www/example.com/www         - your public directory, accessible from www.example.com
/var/www/example.com/application -

index.php

<?php
// for recent php version, __DIR__ just do it
define('PUBLICPATH', __DIR__.DIRECTORY_SEPARATOR);
// in case __DIR__ is not available use the following:
//  define('PUBLICPATH', dirname(__FILE__).DIRECTORY_SEPARATOR);

// I prefer to have the "root dir" not accessible from public dir
define('ROOTROOT', realpath(PUBLICPATH.'..').DIRECTORY_SEPARATOR);
define('APPPATH', realpath(PUBLICPATH.'..'.DIRECTORY_SEPARATOR.'application').DIRECTORY_SEPARATOR);

require(APPPATH.'bootstrap.php');

For classes definitions, you can also use an autoloader (with __autoload or spl_autoload_register ) to load classes without having to include/require their file before.

EDIT: I replaced '/' by DIRECTORY_SEPARATOR to make it work on both window and unix.