s2member: Eine e-mail senden, wenn der Mitgliedslevel geändert wurde

Im Ordner wp-content/mu-plugins/ („must use“ plugins) wird eine Datei s2-role-change-handler.php angelegt.
/wp-content/mu-plugins/s2-role-change-handler.php

In diese Datei wird folgender Code eingetragen:

<?php
add_action('init', 's2_role_change_event_handler::init', 1);

class s2_role_change_event_handler
{
    public static $user_id;
    public static $user_old_role;

    public static function init()
    {
        global $pagenow;

        if (!is_admin()) {
            return; // Not applicable.
        }
        if ($pagenow !== 'user-edit.php') {
            return; // Not applicable.
        }
        if (empty($_REQUEST['user_id'])) {
            return; // Not applicable.
        }
        if (!current_user_can('edit_users')) {
            return; // Not applicable.
        }
        $user = new WP_User((int) $_REQUEST['user_id']);

        static::$user_id       = $user->ID;
        static::$user_old_role = reset($user->roles);

        add_action('set_user_role', 's2_role_change_event_handler::event', 10, 2);
    }

    public static function event($user_id, $new_role)
    {
        if ($user_id === static::$user_id && $new_role !== static::$user_old_role) {
            if (($user = new WP_User($user_id)) && $user->exists()) {
                // Replace $subject, $message, $headers with your information
                $subject = 'User Role Update';
                $message = 'Hello '.$user->first_name.'!'."\r\n"."\r\n".'This is to notify you that your User Role has been changed to "'.$new_role.'".';
                $headers = 'From: Awesome Site <hello@awesomesite.com>'; 
                wp_mail($user->user_email, $subject, $message, $headers);
            }
        }
    }
}

Quelle: https://s2member.com/kb-article/how-do-i-send-an-email-when-the-membership-level-is-changed/