[HOWTO] FCKeditor uploads with Shared Sign-On (and Domain Access)

FCKeditor requires you to set $cookie_domain for its upload functionality (the uploader bootstraps Drupal and needs to be able to read your cookies). Unfortunately, if you have multiple domains using Domain Access and rely on Shared Sign-On for authentication, you can't just set $cookie_domain = 'mydomain.com'

However it seems to work if you automatically generate the $cookie_domain from $_SERVER['HTTP_HOST'], by putting the following two lines in your settings.php

preg_match('/[^.]+\.[^.]+$/', $_SERVER['HTTP_HOST'], $cookie_domain_matches);
$cookie_domain = $cookie_domain_matches[0];

I'm mainly posting this to help anyone else looking for a solution, though I'd be interested to know if there are any downsides to doing this.