Masking Affiliate URL using $post_slug after “/go/”

You’ll want to use a rewrite rule combined with a custom query variable and hooking into template_redirect to catch your /go/... urls.

First up: the rewrite rule. This just maps a regex pattern to an set of query params on index.php. I’m going to use a post ID here rather than a slug. Makes things a bit easier in our catch/redirect function.

add_action('init', 'wpse205416_add_rule');
function wpse205416_add_rule()
{
    add_rewrite_rule(
        '^go/(\d+)/?$',
        'index.php?wpse205416_go=$matches[1]',
        'top'
    );
}

the wpse205416_go bit is significant, we’ll fetch it later. WordPress discards query variables it does’t know about so we have to register it by hooking into query_vars.

add_filter('query_vars', 'wpse205416_add_var');
function wpse205416_add_var(array $vars)
{
    $vars[] = 'wpse205416_go';
    return $vars;
}

And now we can hook into template_redirect and look for our wpse205416_go variable. If present, fetch the post (to make sure it exists) and fetch the URL and redirect. The onlything interesting here is the _wpse205416_not_found function which just makes sure we actually 404. Calling $wp_query->set_404() doesn’t send the correct HTTP status.

function _wpse205416_not_found()
{
    global $wp_query;
    status_header(404);
    $wp_query->set_404();
}

add_action('template_redirect', 'wpse205416_catch_go');
function wpse205416_catch_go()
{
    $id = get_query_var('wpse205416_go');
    if (!$id) {
        return; // not a redirect
    }

    $post = get_post($id);
    if (!$post) {
        return _wpse205416_not_found(); // not a valid post ID, 404 it!
    }

    // whatever your meta key is
    $url = get_post_meta($post->ID, '_wpse205416_go_url', true);
    if (!$url) {
        return _wpse205416_not_found(); // no url, 404 it!
    }

    // always exit after redirecting
    wp_safe_redirect($url, 302);
    exit;
}

If you’re interested in learning more about the rewrite api, I wrote a fairly extensive tutorial about it.