How to use query_posts filter ?

Hello.

My current WP configuration:
– WordPress : 2.9.2 in French
– Theme : 3d-realty.1.0.3 modified and localized
– Plugins : Calendar, cforms, Cimy User Extra Fields, IWG Hide Dashboard, New User Email Setup, Role Manager, RSS Footer, Sidebar Login, Simple:Press Forum, StatPress, WP-Table Reloaded, wpNamedUsers
– Hosted by : OVH
– Site : https://www.reseau-gestalt-ouest.asso.fr/
– Issue : My own plugin generates an error

I use WordPress for a site that does not publish any post, only pages. I would like that my RSS feed contains links to the recently modified pages, but not all; I wish to exclude some pages that have no interest in such a feed.

To do this I have defined a custom field ? rgo_montrer_dans_fil_rss ?. I set its value to “oui” if I want the page to be published in the feed. And I have developed a small plugin (my first one) to apply this filter, but it does not work.

Here is the plugin code:

<?php
/*
Plugin Name: RGO RSS filter
Description: Exclut certaines pages du flux RSS : celles qui ne contiennent pas le champ personnalisé 'rgo_montrer_dans_fil_rss' avec une valeur 'oui'
Author: Gilbert Tordeur
Version: 1.0
*/
if (!function_exists('rgo_rss_filter')) {
   function rgo_rss_filter() {
      if (is_feed()) {
         query_posts('post_type=page&showposts=10&meta_key=rgo_montrer_dans_fil_rss&meta_compare==&meta_value=oui');
      }
   }
   add_action('rss2_head','rgo_rss_filter');
}
?>

For now the activation of my plugin immediatly generates the following error on my admin pages:

Warning: Cannot modify header information – headers already sent by (output started at /homez.330/reseaugen/www/w2/wp-content/plugins/rgo-rss-filter/rgo-rss-filter.php:1) in /homez.330/reseaugen/www/w2/wp-content/plugins/simple-forum/library/sf-ahah-handler.php on line 14

Warning: Cannot modify header information – headers already sent by (output started at /homez.330/reseaugen/www/w2/wp-content/plugins/rgo-rss-filter/rgo-rss-filter.php:1) in /homez.330/reseaugen/www/w2/wp-content/plugins/sidebar-login/sidebar-login.php on line 321

I do not understand this error. How can I fix it?

Thank you in advance,
Gilbert
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