You can use array_multisort()
Try something like this:
foreach ($mdarray as $key => $row) {
// replace 0 with the field's index/key
$dates[$key] = $row[0];
}
array_multisort($dates, SORT_DESC, $mdarray);
For PHP >= 5.5.0 just extract the column to sort by. No need for the loop:
array_multisort(array_column($mdarray, 0), SORT_DESC, $mdarray);