ok, trovato e risolto.
$stmt = $db->prepare("SELECT photo_categories.title as _title, photo_categories.id as _id, photo_categories.dir as _dir , photo.*, DATE_FORMAT(photo.data_reg,'%d/%M/%Y ore %H:%i') as 'data_it' FROM photo
INNER JOIN photo_categories ON photo_categories.id = photo.id_categories WHERE photo.active=1 ORDER BY RAND() LIMIT 1");