Use "../file.php" and "../_inc/file.php".
It gets more complicated if include files themselves want to include files, because they don't know which directory they're being run from so they don't know how many directories to ".." up from.
I usually just use php_include_path to set a directory of standard include files, and also auto_prepend_file to set some PHP variables, including URL and local path for the site's web root.
See also realpath, which may be of use:
http://www.php.net/realpath