Editor can create any new user except administrator

I have set up a WordPress site for a client. The client has the Editor role, however I have installed the Members plugin and given the client the capability to add new users to the WP admin. This is working just fine.

The question I have is that I would like for the client to have the ability to create new user as with the roles of a Contributor, Subscriber, Editor and Author, but NOT Administrator. The new users the client creates should not have the Administrator role. Is it possible to hide this option somehow?

Thanks
Vayu

It’s actually pretty easy. You need to filter into map_meta_caps and stop editors from creating/editing admins, and remove the administrator role from the ‘editable roles’ array. This class, as a plugin or in your theme’s functions.php file would do it:

class JPB_User_Caps {

  // Add our filters
  function __construct(){
    add_filter( 'editable_roles', array($this, 'editable_roles'));
    add_filter( 'map_meta_cap', array($this, 'map_meta_cap'), 10, 4);
  }

  // Remove 'Administrator' from the list of roles if the current user is not an admin
  function editable_roles( $roles ){
    if( isset( $roles['administrator'] ) && !current_user_can('administrator') ){
      unset( $roles['administrator']);
    }
    return $roles;
  }

  // If someone is trying to edit or delete and admin and that user isn't an admin, don't allow it
  function map_meta_cap( $caps, $cap, $user_id, $args ){

    switch( $cap ){
        case 'edit_user':
        case 'remove_user':
        case 'promote_user':
            if( isset($args[0]) && $args[0] == $user_id )
                break;
            elseif( !isset($args[0]) )
                $caps[] = 'do_not_allow';
            $other = new WP_User( absint($args[0]) );
            if( $other->has_cap( 'administrator' ) ){
                if(!current_user_can('administrator')){
                    $caps[] = 'do_not_allow';
                }
            }
            break;
        case 'delete_user':
        case 'delete_users':
            if( !isset($args[0]) )
                break;
            $other = new WP_User( absint($args[0]) );
            if( $other->has_cap( 'administrator' ) ){
                if(!current_user_can('administrator')){
                    $caps[] = 'do_not_allow';
                }
            }
            break;
        default:
            break;
    }
    return $caps;
  }

}

$jpb_user_caps = new JPB_User_Caps();

EDIT

Ok, so I took a look into why it was letting user deletion slip through. It looks like delete_user is handled slightly differently from edit_user; I’ve modified the map_meta_cap method to work around this. I’ve tested on 3.0.3 and this will prevent anybody but administrators from actually deleting, editing, or creating an administrator.

EDIT 2

I updated the code to reflect @bugnumber9 ‘s answer below. Please go give that answer an upvote!