- var_dump: documentation ( source)
<?php
$creationTime = \DateTimeImmutable::createFromFormat('Y-m-d H:i:s', '2017-07-10 10:00:00');
$expirationTime = \DateTimeImmutable::createFromFormat('Y-m-d H:i:s', '2017-07-10 11:00:00');
$time = $creationTime->diff($expirationTime);
var_dump($time->format('s'));