$conf['links'] = array(
    'http://plouic.fr/' => 
        array('label'=>'plouic',
                       'new_window' => true, 
                       'eval_visible' => 'return !is_a_guest();'),           
                        );

<?php
/*
Plugin Name: Plugin Personnel
Version: 1.0
Description: Plugin Personnel
Plugin URI: http://piwigo.org
Author:
Author URI:
*/


add_event_handler('loc_begin_page_header', 'tag_guest_denied', 'MasquerLiensGuest');


function MasquerLiensGuest($menu_ref_arr)
{
    global $user, $conf, $page, $template;
    $menu = & $menu_ref_arr[0];

  if ( is_a_guest() )
  {
  $menu->hide_block('links');

  }
 
}
?>