- var_dump: documentation ( source)
<?php
$date = new DateTime('2015-05-28');
var_dump($date->format('c'));
$date->modify('monday this week midnight');
echo $date->format('c');
$date = new DateTime('2015-05-28');
var_dump($date->format('c'));
$date->modify('monday last week midnight');
echo $date->format('c');