[calorie]

Snip from http://www.php.net/manual/en/function.include.php page:

Files for including are first looked in include_path relative to the current working directory and then in include_path relative to the directory of current script. E.g. if your include_path is ., current working directory is /www/, you included include/a.php and there is include "b.php" in that file, b.php is first looked in /www/ and then in /www/include/. If filename begins with ./ or ../, it is looked only in include_path relative to the current working directory.

So you could set the include_path in php.ini or use ini_set to set it:

ini_set('include_path', '.:../:./subfolder:../subfolder'); // or whatever