- var_dump: documentation ( source)
- date_default_timezone_set: documentation ( source)
<?php
date_default_timezone_set('UTC');
$date = new DateTime('2015-06-23 18:00:00', new DateTimeZone('America/Sao_Paulo'));
var_dump($date);
$date->setTimezone(new DateTimeZone('-03:00'));
var_dump($date);
//var_dump($date->getTimestamp()); // comment out, to get the expected result
$date->setTimezone(new DateTimeZone('America/Sao_Paulo'));
var_dump($date);