- var_dump: documentation ( source)
<?php
$x = new DateTimeImmutable('2015-07-31');
$y = $x->modify('30 day of -1 month');
var_dump($y);
var_dump($y->setDate($y->format('Y'), $y->format('n'), 30));
var_dump($y->setDate((int) $y->format('Y'), (int) $y->format('n'), 30));