Initially I was using strtotime to fetch the desired format, but i was getting the initial time as 01/01/1970, then i realized(with little googling) that using strtotime expects string to be in format of y-m-d do we firstly need to convert this into this format. Below is the code for desired output.
$oldDate = '16-05-12';
$arr = explode('-', $oldDate);
$newDate = (string)($arr.'-'.$arr.'-'.$arr);