public function getAllBill($user_rank, $user_id){

  $sql = 'SELECT * FROM invoice_user
  INNER JOIN users on users.user_id = invoice_user.user_id';

  $sql_clause = 'WHERE user_id = :user_id)';
  $sql_group  = 'GROUP BY invoice_number ORDER BY invoicedate DESC';

  if ($user_rank === 'member') {
    $final_sql = $sql.' '.$sql_clause.' '.$sql_group;
    $parameters = array(':user_id' => $user_id);
  }
  else if ($user_rank === 'moderator'){
    $final_sql = $sql.' '.$sql_group;
    $parameters = array();
  }
  else {
    $error = TRUE;
  }

  if (!isset($error)) {
    $request = $this->bdd->prepare($final_sql);
    $request->execute($parameters);
    return $request->fetchAll();
  } else return array();

}