test script:

$d1 = new DateTime('2012-05-31');
$d2 = new DateTime('2013-03-02');

$diff1 = $d1->diff ($d2);

$diff2 = $d2->diff ($d1);

echo "case 1: ".$diff1->m." months, case 2: ".$diff2->m." months";


expected result:

both $diff1->m and $diff2->m are 9

I can more or less confirm that DateInterval has 
Daylight Savings changes backward, at least for my 
2014 test cases. In the springtime in the US we 
spring forward at 2am on a Sunday morning. 
2am and 3am are the same exact time, and share the 
same exact timestamp. At 3am, 2 actual hours 
have elapsed since midnight.

In the fall we fall backward on a Sunday at 2am. 
In this case, at 3am 4 actual hours have elapsed 
since midnight.

My code shows that DateTime and DateTimezone
handle DST changes correctly -- its DateInterval
that is having the problem.



Here's my test code:

<?php
/**
Daylight Saving Time (United States) 2014 begins at 2:00 AM on
Sunday, March 9
and ends at 2:00 AM on
Sunday, November 2
 */

$tz = new DateTimeZone( 'America/New_York');
$d1 = new DateTime('2014-03-09 00:00:00', $tz);


echo PHP_EOL;
echo 'Springing forward at 2am on Sunday, March 9, 2014...' . PHP_EOL;
for ($n = 0; $n < 7; $n++){
	$d2 = new DateTime(sprintf('2014-03-09 %s:00:00', $n), $tz);
	$ts_diff = ($d2->getTimestamp() - $d1->getTimestamp())/(60 * 60);
	$int = $d2->diff($d1);
	echo 'Comparing ' . $d2->format('g a') . ' to ' . $d1->format('g a') . 
PHP_EOL;
	echo 'Timestamp diff:          ' . $ts_diff . PHP_EOL;
	echo 'Interval diff:           ' . $int->h . PHP_EOL;

}
echo PHP_EOL;
$d1 = new DateTime('2014-11-02 00:00:00', $tz);
echo 'Falling backward at 2am on Sunday, November 2, 2014...' . PHP_EOL;
for ($n = 0; $n < 7; $n++){
	$d2 = new DateTime(sprintf('2014-11-02 %s:00:00', $n), $tz);
	$ts_diff = ($d2->getTimestamp() - $d1->getTimestamp())/(60 * 60);
	$int = $d2->diff($d1);
	echo 'Comparing ' . $d2->format('g a') . ' to ' . $d1->format('g a') . 
PHP_EOL;
	echo 'Timestamp diff:          ' . $ts_diff . PHP_EOL;
	echo 'Interval diff:           ' . $int->h . PHP_EOL;

}

phpinfo(INFO_GENERAL);
?>

Output
========

Springing forward at 2am on Sunday, March 9, 2014...
Comparing 12 am to 12 am
Timestamp diff:          0
Interval diff:           0
Comparing 1 am to 12 am
Timestamp diff:          1
Interval diff:           1
Comparing 2 am to 12 am
Timestamp diff:          2
Interval diff:           3
Comparing 3 am to 12 am
Timestamp diff:          2
Interval diff:           3
Comparing 4 am to 12 am
Timestamp diff:          3
Interval diff:           4
Comparing 5 am to 12 am
Timestamp diff:          4
Interval diff:           5
Comparing 6 am to 12 am
Timestamp diff:          5
Interval diff:           6

Falling backward at 2am on Sunday, November 2, 2014...
Comparing 12 am to 12 am
Timestamp diff:          0
Interval diff:           0
Comparing 1 am to 12 am
Timestamp diff:          1
Interval diff:           1
Comparing 2 am to 12 am
Timestamp diff:          3
Interval diff:           2
Comparing 3 am to 12 am
Timestamp diff:          4
Interval diff:           3
Comparing 4 am to 12 am
Timestamp diff:          5
Interval diff:           4
Comparing 5 am to 12 am
Timestamp diff:          6
Interval diff:           5
Comparing 6 am to 12 am
Timestamp diff:          7
Interval diff:           6
phpinfo()
PHP Version => 5.4.9

System => Darwin Christophers-MacBook-Air.local 12.4.0 Darwin Kernel Version 
12.4.0: Wed May  1 17:57:12 PDT 2013; root:xnu-2050.24.15~1/RELEASE_X86_64 x86_64
Build Date => Dec 11 2012 12:36:33

Yet another test script:

<?php

$zone = new DateTimeZone('Europe/Madrid');
$start = new DateTime('2013-03-31 1:59:00 ', $zone);
$end = new DateTime('2013-03-31 3:00:00', $zone);

$difference = $start->diff($end);
echo $difference->format('%H:%I:%S') . PHP_EOL;

$start->setTimeZone(new DateTimeZone('UTC'));
$end->setTimeZone(new DateTimeZone('UTC'));
$difference = $start->diff($end);
echo $difference->format('%H:%I:%S') . PHP_EOL;

?>

Expected output:

00:01:00
00:01:00

Actual output:

01:01:00
00:01:00

The easy way to see the difference :
http://3v4l.org/ab0Hm#v5424


The code 
<?php

$debut = mktime(0, 0, 0, 10, 27, 2013);
$fin = mktime(23, 59, 59, 11, 10, 2013);

$d1 = new DateTime('now',new DateTimeZone('Europe/Paris'));
$d2 = new DateTime('now',new DateTimeZone('Europe/Paris'));
$d1->setTimestamp($debut);
$d2->setTimestamp($fin);
$diff = $d1->diff($d2);

print_r($diff);

Duplicate of #63442

This looks already fixed in PHP/5.6.0 (2014-08-28) and backport versions released on 2014-09-01: PHP/5.4.24 and PHP/5.5.8

http://3v4l.org/MVWMG

Looks like this has been fixed.