"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

Comienza ejecutando la reproducción de PHP proporcionada contra el comportamiento de DateTimeImmutable::add() y DateInterval descrito en el issue, comparando los intervalos creados por el constructor y por DateTimeImmutable::diff(). Rastrea la implementación de fecha/hora de PHP responsable de las transiciones de DST; se considera terminado cuando intervalos equivalentes de tres horas produzcan las marcas de tiempo esperadas en ambos casos de cambio de DST, con cobertura de regresión para los escenarios demostrados.