"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

まず、issue で説明されている DateTimeImmutable::add() と DateInterval の動作に対して、提供された PHP の再現コードを実行し、コンストラクターと DateTimeImmutable::diff() によって作成された間隔を比較します。DST の遷移を担当する PHP の日付/時刻実装を追跡します。両方の DST 切り替えケースで、同等の 3 時間の間隔が期待されるタイムスタンプを生成し、実証されたシナリオに対するリグレッションカバレッジがあれば完了です。