PHP (Month - Date - Year) strtotime method not working

Question

This is my Date : 05-03-2021 (Month-Date-Year). When i use following query its return wrong date format.

$originaldate="05-03-2021";

$t_date=date('Y-m-d',strtotime($originaldate));

echo $t_date;

Expected : 2021-05-03;

But Result : 2021-03-05;