- assert: documentation ( source)
<?php
$originalDate = new DateTimeImmutable('2011-11-11');
$modifiedDate = $originalDate->modify('+1 year');
assert($originalDate !== $modifiedDate);
assert($modifiedDate->format('Y-m-d') === '2012-11-11');
assert($originalDate->format('Y-m-d') === '2011-11-11');