DateInterval issues with negative directions

Description

I am going to put a few connected issues I have noticed around DateInterval and DatePeriod into a single report:

1: DateInterval->invert is "readonly"

DateInterval:::__construct() only allows positive $duration e.g. P1D not P-1D (I appreciate this is the correct behaviour as it is based around the ISO8601 which has no concept of negative periods). However... the DateInterval->invert which allows the period to be reversed can only be changed by manually setting that property and the docs explicitly say:

The available properties listed below depend on PHP version, and should be considered as readonly.

It would be good to be able to change this property safely.

2: DateInterval::createFromDateString allows negative periods

DateInterval::createFromDateString('-1 day') is accepted but this sets DateInterval->d = -1 rather than using the ::invert property. This is inconsistent with the always-positive periods which __construct allows.

I would expect it to:

• Reject mixed positive and negative period values, what would +1 year -6 month even mean?
• Accept if only negative periods passed but then set ::invert = 1 and the properties of ::y..f as +ve

3: DateInterval::createFromDateString('-1 day') leads to infinite loop in DatePeriod

$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-01'),
    \DateInterval::createFromDateString('-1 day'),
    new \DateTimeImmutable('2000-01-05'),
);

echo iterator_count($datePeriod); // stuck in infinite loop

4: DateInterval::createFromDateString('-1 day') still doesn't work if start and end reversed in DatePeriod

$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-05'),
    \DateInterval::createFromDateString('-1 day'),
    new \DateTimeImmutable('2000-01-01'),
);

echo iterator_count($datePeriod); // 0 (but would expect 4)

5: DatePeriod doesn't respect DateInterval->invert

$dateInterval = new \DateInterval('P1D');
$dateInterval->invert = 1;

# will not go backwards from high to low dates
$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-05'),
    $dateInterval,
    new \DateTimeImmutable('2000-01-01'),
);

echo iterator_count($datePeriod); // 0 (but would expect 4)

# incorrectly goes forward from low to high dates
$datePeriod = new \DatePeriod(
    new \DateTimeImmutable('2000-01-01'),
    $dateInterval,
    new \DateTimeImmutable('2000-01-05'),
);

echo iterator_count($datePeriod); // 4 (but would expect 0)

PHP Version

8.3.114

Operating System

Rocky 8

Beginne damit, die Beispiele mithilfe der Einstiegspunkte DateInterval und DatePeriod nachzustellen, einschließlich negativer Intervalle, umgekehrter Datumsangaben und der invert-Eigenschaft. Überprüfe das bestehende Verhalten von Datumsperioden und entscheide über konsistente Semantik für negative Richtungen, gemischte Vorzeichen und invertierte Intervalle. Erledigt ist dies, wenn die gemeldeten Fälle ein definiertes, nicht endlos laufendes Verhalten und eine Regressionstestabdeckung haben.