- var_dump: documentation ( source)
<?php
$tz=new DateTimeZone('UTC');
$date = new DateTime('2000-12-31', $tz);
$date->add(new DateInterval('P1M'));
var_dump($date->format('Y-m-d'));
$date->add(new DateInterval('P1M'));
var_dump($date->format('Y-m-d'));