<?php
$a=array("John"=>array("test_id"=>1, "student_id"=>2, "mark"=>3),
    "Peter"=>array("test_id"=>1, "student_id"=>4, "mark"=>1),
    "Hans"=>array("test_id"=>1, "student_id"=>3, "mark"=>1),
    "Helen"=>array("test_id"=>2, "student_id"=>2, "mark"=>5));

$b=array_values($a);


$c = array_keys($a);

$i = count($b);
  for ($j = 0; $j<$i; $j++)
  {
      if ($b[$j]['mark']>$b[$j+1]['mark'])
      {
         $max = $b[$j];
          $b[$j] = $b[$j+1];
          $b[$j+1]=$max;
//_____________________________________________
          $max = $c[$j];
          $c[$j] = $c[$j+1];
          $c[$j+1]=$max;


      }


  }
$bc = array_combine($c, $b);
print_r($bc);


// your code goes here