Description:
------------
DateTime::add() behaves incorrectly when adding times to a DateTime representing a point during the hour that is repeated during a transition from daylight savings time to standard time. This problem seems to arise only in cases where the associated DateTimeZone has been defined using a geographical label like 'America/Boise'.

One thing that is especially surprising in these cases is that using DateTime::add() to add an hour to a time can cause the description of the timezone that is reported by the 'T' format option to change in such a way that the operation appears to have succeed as intended while the call to DateTime::add() actually leaves the underlying Unix timestamp entirely unchanged. What this means is that there is a bug in DateTime::format(), as well as in DateTime::add().

I have verified that this problem occurs in PHP 7.3.21 and PHP 7.4.0.

Test script:
---------------
$dt = new DateTime('@1604215800');
$dt->setTimezone(new DateTimeZone('America/Boise'));
echo "{$dt->format('Y-m-d H:i:s T')} | {$dt->getTimestamp()}\r\n";
$dt->add(new DateInterval('PT1H'));
echo "{$dt->format('Y-m-d H:i:s T')} | {$dt->getTimestamp()}\r\n";
$dt->add(new DateInterval('PT1H'));
echo "{$dt->format('Y-m-d H:i:s T')} | {$dt->getTimestamp()}\r\n";
$dt->add(new DateInterval('PT1M'));
echo "{$dt->format('Y-m-d H:i:s T')} | {$dt->getTimestamp()}\r\n";


Expected result:
----------------
2020-11-01 01:30:00 MDT | 1604215800
2020-11-01 01:30:00 MST | 1604219400
2020-11-01 02:30:00 MST | 1604223000
2020-11-01 02:31:00 MST | 1604223060


Actual result:
--------------
2020-11-01 01:30:00 MDT | 1604215800
2020-11-01 01:30:00 MST | 1604215800
2020-11-01 01:30:00 MST | 1604215800
2020-11-01 01:31:00 MDT | 1604215860