SELECT COUNT(*)
 FROM   `spip_signatures` AS signatures,
     `spip_petitions` AS petitions,
     `spip_articles` AS articles
 WHERE (signatures.id_article = '2666')
     AND (petitions.id_article=signatures.id_article)
     AND (petitions.id_article=articles.id_article)

retourne 0 signatures comme il le devrait mais

 SELECT COUNT(*)
 FROM   `spip_signatures` AS signatures,
     `spip_petitions` AS petitions,
     `spip_articles` AS articles
 WHERE (signatures.id_article = '2666')
     AND (petitions.id_article=articles.id_article)
     AND (petitions.id_article=signatures.id_article)

retourne toutes les signatures du site.
Le premmier qui dit que AND est commutatif a perdu.