Always send user to primary (WPMU) site

Users of a WordPress Multisite can configure their Primary Site, when they click the My Sites link in the WP Admin Bar as shown below.

By default, WPO365 will not take a user's primary site in consideration, because of the complexity to decide whether or not users should be redirected to their primary sites or not. However, you can override this behavior by adding the following piece of PHP code to your theme's functions.php file.

add_filter('login_redirect', function ($redirect_to, $requested_redirect_to, $wp_usr) {

    if (is_multisite() && is_user_logged_in() && $wp_usr->has_cap('read')) {
        $primary_blog = get_user_meta($wp_usr->ID, 'primary_blog', true);
        switch_to_blog($primary_blog);
        $site_url = get_home_url();
        restore_current_blog();
        return $site_url;
    }

    return $redirect_to;
}, PHP_INT_MAX, 3);