Multisite: Redirect to appropriate domain if subdomain doesn’t exist

If you want all unregistered subdomains to redirect to the main domain site:

function prevent_multisite_signup() 
{
    $domain = $_SERVER['HTTP_HOST'];
    $domain = explode('.', $domain);
    $domain = array_reverse($domain);
    $domain = "$domain[1].$domain[0]";
    wp_redirect($domain, 301);
    exit;
}
add_action('signup_header', 'prevent_multisite_signup');