Scusate ragazzi, forse sarà l'ora tarda, ma se eseguo questo codice
$sql = "SELECT * FROM products_attributes WHERE products_id ='$pid' AND options_id = '1' AND options_values_id = '$prod'";
$rows = $db->query($sql);
while ($r = $db->fetch_array($rows)){
$p = $r[products_id];
}...