datetime to string php

<?php
$date = new DateTime('2000-01-01');
echo $date->format('Y-m-d H:i:s');
?>

$s = '06/10/2011 19:00:02';
$date = strtotime($s);
echo date('d/M/Y H:i:s', $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);   

phpCopy$theDate    = new DateTime('2020-03-08');
echo $stringDate = $theDate->format('Y-m-d H:i:s');

//output: 2020-03-08 00:00:00