Description:
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Translating from a timezone to itself does not return 0.

Reproduce code:
---------------
<?php

    $dateTimeZone1 = new DateTimeZone ("Europe/London");
    $dateTimeZone2 = new DateTimeZone ('Europe/London');
    $dateTime1 = new DateTime ("now", $dateTimeZone1);
    $dateTime2 = new DateTime ("now", $dateTimeZone2);
    echo $dateTimeZone1->getOffset($dateTime2) . "\n";
    echo $dateTimeZone2->getOffset($dateTime1) . "\n";

?>

Expected result:
----------------
0
0

Actual result:
--------------
3600
3600