Solved. Thanks for the suggestion!
On login_page.inc.php, I changed my code to :
include_once ('../includes/config.inc.php');
require_once (MYSQL);
There was a side effect of this code change in my site. My login page has a limited number of users and so I have a select box of user's names which is queried from the database. If someone puts in the wrong password, Larry's original script shows the error and then below it the form again. Well, my form displayed but the select box collapsed because it needed a MySQL connection but can no longer get it because of the "require_once." So I replaced the display of the form with a link "back" to the original page.
I mention this for any other newbies out there.