Description:
------------
DateTime::modify() ignores the timezone (when using the RFC 2822 format for instance).

The right timestamp should be $t1 (from the example below).

Test script:
---------------
<?php
$str = "Tue, 10 Apr 2012 11:27:56 +0300";

$d1 = new DateTime($str);
$d2 = new DateTime;
$d2->modify($str);

$t1 = $d1->getTimestamp();
$t2 = $d2->getTimestamp();
$tz1 = $d1->getTimezone()->getName();
$tz2 = $d2->getTimezone()->getName();
$f1 = $d1->format(DateTime::RFC2822);
$f2 = $d2->format(DateTime::RFC2822);

var_dump(compact('t1', 't2', 'tz1', 'tz2', 'f1', 'f2'));
/*
array(6) {
  't1' =>
  int(1334046476)
  't2' =>
  int(1334050076)
  'tz1' =>
  string(6) "+03:00"
  'tz2' =>
  string(13) "Europe/Vienna"
  'f1' =>
  string(31) "Tue, 10 Apr 2012 11:27:56 +0300"
  'f2' =>
  string(31) "Tue, 10 Apr 2012 11:27:56 +0200"
}
*/

Expected result:
----------------
$t1 == $t2

Actual result:
--------------
$t1 != $t2