register_sidebar_widget problem

I have read article about register_sidebar_widget() function.
But when I tried, I encounterd issue.
I created a.php file, and then have written following code.

register_activation_hook(dirname(__FILE__)."/install.php", 'widget_flynl_activate');

install.php file includes widget_flynl_activate() function.
But when I tested, it did not work.

When I tried this code, it worked well.

register_activation_hook(__FILE__, 'widget_flynl_activate');

In this case, a.php has widget_flynl_activate() function.

Can anyone help me what the reason is here?
Thanks.