"ajax": {
            "method" : "POST",
            "url" : "listar.php"

function lista_usuarios2(){
    global $db,  $username, $usua, $mes, $accion, $res;
  /* variables */

   $query = "  SELECT * FROM users";

         $c = $db->query($query);
         while($data = $c->fetch_array(MYSQLI_ASSOC))
          {
    $arreglo ["data"][] = $data;
  }
  echo json_encode($arreglo);
}