My b2evolution Version: Not Entered

Can anyone help with this -

I need to change the cookie domain for the setCookie() call in _main.inc.php. The code is:
setcookie( 'cookie'.$instance_name.'pass', '', 200000000, $cookie_path, $cookie_domain);
but changing this to:
setcookie( 'cookie'.$instance_name.'pass', '', 200000000, $cookie_path, '.mydomain.com');
does not work as I thought it would. Anyone have any experience with this?