If you don't specify a path in your include() statement then the file is assumed to be in the same directory. If not you can use other relative path tricks. For example, if "fnctpage.php" is in one subfolder down from your script called "includes" then you can do:
include ('./includes/fnctpage.php');
$_SERVER[‘DOCUMENT_ROOT’] and most other PHP predefined environmental variables will be available in all OS's to create a dynamic absolute path.
You can also specify an "include_path" in your php.ini file to search common directories for included files.