{"user":[{"mac":"60:11:94:08:3C:5D","data":"2018\/04\/04 19:23:01"},{"mac":"60:01:94:44:12:B9","data":"2018\/04\/05 12:22:00"}]}

    $datas = file_get_contents('reg.json');
      $data = json_decode($datas, true);
      foreach ($data["user"] as $key => $element) {
       if ($element['mac'] == $mac) {
        unset($data["user"][$key]);
       }
      }
      file_put_contents("reg.json", json_encode($data), LOCK_EX);

{"user":{"0":{"mac":"60:11:94:08:3C:5D","data":"2018\/04\/04 19:23:01"},"2":{"mac":"60:01:94:44:12:B9","data":"2018\/04\/05 12:22:00""}}

$data["user"] = array_values($data["user"]);