Description:
------------
See the snippet http://3v4l.org/YgUIV
diff() behaves correctly, but add/sub - not.

PHP 5.4.34-5.4.42 and 5.5.18-5.6.10 are affected
As for older versions showing 1 hour difference - does php use its own bundled tzdata instead of system one? It may explain

The dates in example are around latest Europe/Moscow transition from +04:00 to +03:00 happened at 2014-10-25T22:00:00 UTC (01:59:59+04:00 + 1sec -> 01:00:00+03:00)

Test script:
---------------
$tz=new DateTimeZone('Europe/Moscow');
$d1=new DateTime('2014-10-26T00:59:59', $tz);
$d2=new DateTime('2014-10-26T01:59:59', $tz);

var_dump($d2->diff($d1, true)->h);
var_dump($d1->add($int=new DateInterval('PT2H'))->format(DateTime::ISO8601), $d2->sub($int)->format(DateTime::ISO8601));

Expected result:
----------------
Consistent add/sub results assuming 2hrs difference between test dates

2014-10-26T01:59:59+0300 for add()
2014-10-26T00:59:59+0400 for sub()

Actual result:
--------------
http://3v4l.org/YgUIV

diff=2
2014-10-26T02:59:59+0300
2014-10-25T23:59:59+0400