Description:
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DateTime::diff return wrong result for first day of month.
Tested on PHP Version 5.4.10 \ 5.4.15

Test script:
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<?php
date_default_timezone_set('Europe/Moscow');

$datetime1 = new DateTime('13-03-01');
$datetime2 = new DateTime('13-04-01');

$datetime3 = new DateTime('13-03-02');
$datetime4 = new DateTime('13-04-02');

$interval = $datetime2->diff($datetime1);
echo $interval->format('%m month, %d days'), '<br>'; //1 month, 3 days

$interval = $datetime4->diff($datetime3);
echo $interval->format('%m month, %d days'); //1 month, 0 days

Expected result:
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1 month, 0 days
1 month, 0 days

Actual result:
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1 month, 3 days
1 month, 0 days