動的ピボットテーブルに最小値と最大値を表示する方法

次の問題のどの部分を解決できませんか？

<?php

include('path/to/connection/statme.nts');

$query = "
SELECT id_u
     , id_k
     , k
  FROM dt_table
 ORDER 
    BY id_u
     , id_k;
";

$result = mysqli_query($conn,$query);

$array = array();

while($row = mysqli_fetch_assoc($result)){
  $array[] = $row;
}

$new_array = array();

foreach($array as $v){
  $new_array[$v['id_u']][] = $v['k'];
}

print_r($new_array);
?>