Generate user login in WordPress - function example

userlogingenerate.php

<?php
 
/**
* Generate login for new WP user
* 
* return string uniq login
*/
function generate_new_userlogin(){
  $users_ids = get_users('fields=ID&number=3&orderby=registered&order=DESC');
  $last_id = max($users_ids);
  $new_id = $last_id+1;
  $user_login = 'u'. $new_id;
  return $user_login;
}

//Use 
$user = register_new_user( $user_login = generate_new_userlogin(), $user_email = 'name@site.example' );

?>