我需要通过getNotifications函数中的值’start_date’DESC顺序对通知数组进行排序：

$posts_id_arr = getPoststIds($conn);

foreach ($posts_id_arr as $key => $val) {
  $total_arr[$key] = [
    'notification' => getNotifications($val['post_id'],$user_id,$conn)
  ];
}

$response_array = array('data' => $total_arr,'more things' => $more_array);
echo json_encode($response_array);

现在由于foreach循环,订单是post id.

data {
       notification: 
      [
       {
        post_id: “1",start_date: "2016-10-10 08:00:00",},{
        post_id: “1",start_date: "2016-10-10 12:00:00",}
    ],notification:
      [
        post_id: “2",start_date: "2016-10-10 09:00:00",{
        post_id: “2",start_date: "2016-10-10 13:00:00",}
    ]
}

我需要它是1：08：00,2：09：00,1：12：00,2：13：00

foreach ($points as $key => $val) {
    $time[$key] = $val[0];
}

array_multisort($time,SORT_ASC,$points);