Subdomain(s) and S_SERVER['HTTP_HOST']

You can use this code
$allowed_domains = array('domain1.tld', 'domain2.tld');
$default_domain  = 'domain1.tld';

if (in_array($_SERVER['HTTP_HOST'], $allowed_domains, TRUE))
{
        $domain = $_SERVER['HTTP_HOST'];
}
else
{
        $domain = $default_domain;
}

if ( ! empty($_SERVER['HTTPS']))
{
        $config['base_url'] = 'https://'.$domain;
}
else
{
        $config['base_url'] = 'http://'.$domain;
}