"Same" DateInterval instances lead to different results, depending on the origin (in combination with DST-switches)

Description

The following code:

<?php
function getIntervalSpecificationFromDateInterval(DateInterval $interval): string
{
    $specification = '';

    if ($interval->invert) {
        $specification .= '-';
    }

    $specification .= 'P';
    $specification .= $interval->y ? $interval->y . 'Y' : '';
    $specification .= $interval->m ? $interval->m . 'M' : '';
    $specification .= $interval->d ? $interval->d . 'D' : '';

    if ($interval->h + $interval->i + $interval->s) {
        $specification .= 'T';
        $specification .= $interval->h ? $interval->h . 'H' : '';
        $specification .= $interval->i ? $interval->i . 'M' : '';
        $specification .= $interval->s ? $interval->s . 'S' : '';
    }

    if ($specification === 'P' || $specification === '-P') {
        $specification = 'PT0S';
    }

    return $specification;
}

$timezone   = new DateTimeZone('Europe/Berlin');
$dstSwitchA = new DateTimeImmutable('2025-03-30 01:00:00', $timezone);
$dstSwitchB = new DateTimeImmutable('2025-10-26 01:00:00', $timezone);
$durationA  = new DateInterval('PT3H');
$durationB  = (new DateTimeImmutable('2025-03-28 01:00:00', $timezone))->diff(
    new DateTimeImmutable('2025-03-28 04:00:00', $timezone),
    true
);
$durationC  = (new DateTimeImmutable('2025-03-28 01:00:00', $timezone))->diff(
    new DateTimeImmutable('2025-03-28 04:00:00', $timezone),
);
$durationD  = (new DateTimeImmutable('2025-03-28 01:00:00', $timezone))->diff(
    new DateTimeImmutable('2025-03-28 04:00:00', $timezone),
);
$durationD  = new DateInterval(getIntervalSpecificationFromDateInterval($durationD));

// Test 1st DST-switch (+1 hour)
echo 'TEST 1 (DST-switch +1 hour)' . PHP_EOL;
$datetimeA = $dstSwitchA->setTimezone($timezone)->add($durationA);
echo 'Constructed:     ' . $datetimeA->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

$datetimeB = $dstSwitchA->setTimezone($timezone)->add($durationB);
echo 'Diff (absolute): ' . $datetimeB->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

$datetimeC = $dstSwitchA->setTimezone($timezone)->add($durationC);
echo 'Diff (relative): ' . $datetimeC->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

$datetimeD = $dstSwitchA->setTimezone($timezone)->add($durationD);
echo 'Workaround:      ' . $datetimeD->format('Y-m-d H:i:s') . PHP_EOL; // expected: 2025-03-30 05:00:00

echo PHP_EOL;

echo 'TEST 2 (DST-switch -1 hour)' . PHP_EOL;

// Test 2nd DST-switch (-1 hour)
// Expected value: 2025-10-26 03:00:00
$datetimeA = $dstSwitchB->setTimezone($timezone)->add($durationA);
echo 'Constructed:     ' . $datetimeA->format('Y-m-d H:i:s') . PHP_EOL;

$datetimeB = $dstSwitchB->setTimezone($timezone)->add($durationB);
echo 'Diff (absolute): ' . $datetimeB->format('Y-m-d H:i:s') . PHP_EOL;

$datetimeC = $dstSwitchB->setTimezone($timezone)->add($durationC);
echo 'Diff (relative): ' . $datetimeC->format('Y-m-d H:i:s') . PHP_EOL;

$datetimeD = $dstSwitchB->setTimezone($timezone)->add($durationD);
echo 'Workaround:      ' . $datetimeD->format('Y-m-d H:i:s') . PHP_EOL;

Resulted in this output:

TEST 1 (DST-switch +1 hour)
Constructed:     2025-03-30 05:00:00
Diff (absolute): 2025-03-30 04:00:00
Diff (relative): 2025-03-30 04:00:00
Workaround:      2025-03-30 05:00:00

TEST 2 (DST-switch -1 hour)
Constructed:     2025-10-26 03:00:00
Diff (absolute): 2025-10-26 04:00:00
Diff (relative): 2025-10-26 04:00:00
Workaround:      2025-10-26 03:00:00

But I expected this output instead:

TEST 1 (DST-switch +1 hour)
Constructed:     2025-03-30 05:00:00
Diff (absolute): 2025-03-30 05:00:00
Diff (relative): 2025-03-30 05:00:00
Workaround:      2025-03-30 05:00:00

TEST 2 (DST-switch -1 hour)
Constructed:     2025-10-26 03:00:00
Diff (absolute): 2025-10-26 03:00:00
Diff (relative): 2025-10-26 03:00:00
Workaround:      2025-10-26 03:00:00

PHP Version

PHP 8.3.16 (cli) (built: Jan 14 2025 20:07:09) (NTS Visual C++ 2019 x64)
Copyright (c) The PHP Group
Zend Engine v4.3.16, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.16, Copyright (c), by Zend Technologies
    with Xdebug v3.4.1, Copyright (c) 2002-2025, by Derick Rethans

Tested with PHP 8.1.0 to 8.4.8

Operating System

No response

Commencez par exécuter la reproduction PHP fournie avec le comportement de DateTimeImmutable::add() et DateInterval décrit dans l’issue, en comparant les intervalles créés par le constructeur et par DateTimeImmutable::diff(). Suivez l’implémentation de date/heure de PHP responsable des transitions DST ; le travail est terminé lorsque des intervalles équivalents de trois heures produisent les horodatages attendus dans les deux cas de changement de DST, avec une couverture de régression pour les scénarios présentés.