- var_dump: documentation ( source)
- strtotime: documentation ( source)
<?php
$interval = new DateInterval('P1D');
$dt = new DateTime('2012-05-05');
$dt->modify('first day of this month');
$dt->setTimestamp(
strtotime('first day of this month', $dt->getTimestamp())
);
var_dump($dt);
$dt->add($interval);
var_dump($dt);