public function get_all($order_by = null)
 {
     $enquiry_list = array();
     $this->db->connect();
     $sql = "SELECT * from tbl_categories";
     if (!is_null($order_by)) {
         $sql = $sql . " ORDER BY " . $order_by;
     }
     $result = $this->db->fetchQuery($sql);
     while ($row = $result->fetch_assoc()) {
         $enquiry = new Enquiry();
         $enquiry->set_id($row['id']);
         $enquiry->set_category_name($row['category_name']);
         $enquiry->set_added_date($row['added_date']);
         $enquiry->set_status($row['status']);
         $enquiry_list[] = $enquiry;
     }
     $this->db->close();
     return $enquiry_list;
 }