php change date format from d-m-y to y-m-d

$date = DateTime::createFromFormat('d/m/Y', "24/04/2012");
echo $date->format('Y-m-d');

$var = '20/04/2012';
$date = str_replace('/', '-', $var);
echo date('Y-m-d', strtotime($date));

$old_date = date('l, F d y h:i:s');              // returns Saturday, January 30 10 02:06:34
$old_date_timestamp = strtotime($old_date);
$new_date = date('Y-m-d H:i:s', $old_date_timestamp);