图片相册.选择不起作用

我试图用下一个/上一个图像制作相册。显示第一个图像(else语句(，但是当我按下一个/最后一个时，图像URL没有切换。我该如何解决这个问题？

$next= $_POST['next'];
$last= $_POST['last'];
if($last== "last" ){
  $result=mysqli_query($con, "SELECT * FROM album WHERE user_id='$id' AND (time_added>'$time') ORDER BY time_added DESC LIMIT 1");
  $row=mysqli_fetch_array($result);
  $time = $row['time_added']; //Y-m-d H:i:s
  $url = $row['img_url'];
}
else if($next== "next"){
  $result=mysqli_query($con, "SELECT * FROM album WHERE user_id='$id' ORDER BY time_added DESC AND (time_added<'$time') LIMIT 1");
  $row=mysqli_fetch_array($result);
  $time = $row['time_added'];
  $url = $row['img_url'];
}
else{
  $result=mysqli_query($con, "SELECT * FROM album WHERE user_id='$id' ORDER BY time_added DESC LIMIT 1");
  $row=mysqli_fetch_array($result);
  $url = $row['img_url'];
  $time = $row['time_added'];
}