path - including php files in a header that is itself included in files in different folder levels -

ok, title little confusing structure of web site:

/ |--header.php |--page1.php +--css    |--style.css +--subsection    |--page2.php +--functions    |--classes.php 

right, both page1.php , page2.php include header.php. using relative file path whatever php page header.php.

header.php includes other files; example css/style.css , functions/classes.php.

i might include them following:

<link rel="stylesheet" href="css/style.css">  <?php require_once("functions/classes.php"); ?> 

this work page1.php reletive paths correct. subsection/page2.php fail paths should ../css/style.css , ../functions/classes.php remain defined in header.php.

my question how can header.php use correct relative file paths includes regardless of file calling header.php (e.g. apage.php) located in web site directory.

set base path css /functions :

define('css_base','insert/full/path/here'); 

then access css in header.php using

<link rel="stylesheet" href="<?=css_base;?>/style.css">