php add days to date

$date = "Mar 03, 2011";
$date = strtotime($date);
$date = strtotime("+7 day", $date);
echo date('M d, Y', $date);

$start_date = "2015/03/02";  
$date = strtotime($start_date);
$date = strtotime("+7 day", $date);
echo date('Y/m/d', $date);