From login.inc.php (v6.2.3 r400):
$user = (isset($_POST['user']) ? $_POST['user']
: (isset($_GET['user']) ? $_GET['user']
: (isset($_SERVER['PHP_AUTH_USER']) ? $_SERVER['PHP_AUTH_USER']
: "")));
(and similar for password)
This has an undesirable side effect: if a user uses the same username/password to authenticate through .htaccess/.htpasswd AND php addressbook, it is impossible to logout of php addressbook, for instance to switch to a different user. It took me a good while to figure out why I couldn't logout.:-/
Anonymous
So, you get "Single-Sign-On" nearly for free :-).
I see your point, but it’s kind of impractical if you want to log in and out with multiple user acounts. Forces you to quit and launch the browser every time.