(PHP 5 >= 5.2.0)
DateTime::getOffset — Returns the daylight saving time offset
Returns DST offset in seconds on success or FALSE on failure.
Example #1 Comparing offsets between Summer and Winter
<?php
date_default_timezone_set('Europe/London');
$winter = new DateTime('2008-12-25 14:25:41');
$summer = new DateTime('2008-07-14 14:25:41');
echo $winter->getOffset(); // Winter offset: 0
echo $summer->getOffset(); // Summer offset: 3600 = 1 hour
?>