require_once('') relative path problem

Hi, I have problem in undertanding the require_once/require/include/include_once functionality. Php files in my project are located in different folders. Some folders have common files for other files. And file layout is like this /* File: a.php */ require_once('../test/db.php'); ... /* File: db.php */ require_once('../globals/global.php'); ... when i include a.php in another file i get error for global.php file that path could not be found. And if i correct the path in db.php then someother file which is using db.php will get error. So, can someone elaborate that how relative path works in require_once. what is the starting directory for require_once function which is using relative path. Or Is there any way that i can included some constant which include the absolute path from which all relative paths can be determined? Comments welcome.