PHP test sorting on sub-document

sort_test.php

<?php

try{
  $m = new Mongo('localhost');
  $db = $m->abc;
  $collection = $db->def;
  $collection->drop();

  // Insert test data
  $collection->insert( array('A' => 1, 'B' => 4, 'C' => array("C1" => 'val', "C2" => 15) ) );
  $collection->insert( array('A' => 2, 'B' => 5, 'C' => array("C1" => 'val', "C2" => 5) ) );
  $collection->insert( array('A' => 3, 'B' => 6, 'C' => array("C1" => 'val', "C2" => 10) ) );

  $cursor = $collection->find(array(), array('C.C2'))->sort(array('C' => -1))->limit(1);
  foreach($cursor as $data){
    print($data['C']['C2']);
  }

}
catch(Exception $e){
  print_r($e->getMessage());
  print("\n");
}

?>