Description:
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If I currently have datetime in DST, and then add some interval to that date (to skip DST), invalid datetime is returned.

How it is: 2013-10-27 01:45 (in DST) + 30 min = 02:15 (not in DST)
How it should be: 2013-10-27 01:45 (in DST) + 30 min = 01:15 (not in DST)

Same for all php versions about >= 5.2.0 : http://3v4l.org/DUO06

Test script:
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$tz = new DateTimezone('Europe/Berlin'); # uses DST
$dt = new DateTime('2013-10-27 01:45:00', $tz);
echo $dt->format('Y-m-d H:i I') . "\n";
$dt->modify("+30 minute");
echo $dt->format('Y-m-d H:i I') . "\n";

Expected result:
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2013-10-27 01:45 1
2013-10-27 01:15 0

Actual result:
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2013-10-27 01:45 1
2013-10-27 02:15 0