wpdb and include() questions

hi, I am new to WP programming and I am confused about how to properly use the WP include files and how to use $wpdb.

I’d like to call get_user_by() and wp_check_password() from a PHP script that runs standalone. That is, the script is not invoked by any of my wordpress pages.

I figure I’ll just include whatever file that’s missing, so I ended up adding the following one by one:

require_once(‘wp-includes/plugin.php’);
require_once(‘wp-includes/pluggable.php’);
require_once(‘wp-includes/capabilities.php’);
require_once(‘wp-includes/formatting.php’);
require_once(‘wp-includes/cache.php’);

and then I hit the wall — Fatal error: Call to a member function get() on a non-object in /home6/mymuscl3/public_html/wp-includes/cache.php on line 113

ok, wrong approach. After reading up the documentation of get_user_by(), I discovered $wpdb. So I guess I am supposed to do this:

require_once(‘wp-includes/wp-db.php’);
globalize $wpdb;

but the thing is, wp-db.php only defines the wpdb class. Where is the global variable $wpdb declared? Should I use wpdb(…) as a constructor directly? And if so, how to use $mywpdb = wpdb(…) to call get_user_by()?