hi there.
so long as there isnt another file/page with 'login' as a name then it *should* automatically go through to the login.php page.
For example, if you go to www.motley.co.uk/contact it automatically takes you through to the contact.php page without having the .php extension actually used in the url
if that isnt sufficient for your needs, then you would need to look at using a .htaccess file and setting up a redirect rule
Jamie