And of course your config file could simply define an include path:
define('APP_BASE_PATH', '/home/username/public_html/app_name');
set_include_path(APP_BASE_PATH.'/includes/' . PATH_SEPARATOR . get_include_path());
Then in your application, to include a file in the "includes" directory:
include "file_name.php";
To include a file in a subdirectory under the includes directory:
include "subdir/filename.php";