Description:
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getTimestamp() is supposed to return the time in UNIX-timestamp. To my 
understanding a unix timestamp is always in UTC and not affected by the timezone.

That is; regardless the the timezone, getTimestamp() should return the same value 
for any date. In most cases it does, but not on daylight-saving-time (DST) 
transitions.

I have create a short script that illustrates the problem.

Test script:
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$date1 = new DateTime("@1351382400"); // Sun Oct 28 2012 02:00:00 GMT+2 (DST)
var_dump($date1->getTimestamp()); //prints: 1351382400
$date1->setTimezone(new DateTimeZone("Europe/Stockholm"););
var_dump($date1->getTimestamp()); //prints: 1351386000