Decreasing DateInterval not producing result

Question

Trying to get the last four Sundays, decrementing in a loop starting with the most recent Sunday first.

// most recent sunday
$mostRecentSunday = new DateTime('last sunday');

// four Sundays ago
$maxDateAttempt = clone $mostRecentSunday;
$maxDateAttempt->modify('-4 weeks');

// interval of one week (same as 7 days or "P7D")
$dateInterval = new DateInterval('P1W');

// isn't this supposedly supposed to switch the increasing interval to decreasing?
$dateInterval->invert = 1;

$dateRange = new DatePeriod($mostRecentSunday, $dateInterval, $maxDateAttempt);

foreach ($dateRange as $day) {
    echo $day->format('F j, Y');
}

Taking @hijarian's answer in this similar question, I thought setting the invert property would solve this, but I cannot get it to work. Then this comment in the PHP docs claims the DatePeriod class isn't even compatible with negative intervals. Anyone have some clarity in the issue? Maybe the PHP docs could use some improvement here.

http://php.net/manual/en/datetime.sub.php – Sammitch Feb 16 '18 at 19:40 — Sammitch, Feb 16 '18 at 19:40

Don't Panic · Accepted Answer · 2018-02-16T20:15:08.990

That comment in the PHP docs is only partially correct. Everything I've read and experimented with so far seems to indicate that DatePeriod doesn't work with negative DateIntervals when using an end date. Maybe there's some initial check that the minimum is less than the maximum before it does anything, but I'm really not sure why it doesn't work.

However, it does work if you use the recurrences constructor rather than setting an end date.

$dateRange = new DatePeriod($mostRecentSunday, $dateInterval, 3);
// using 3 rather than 4 because the initial value is one occurrence

But you have to create your DateInterval like this instead:

$dateInterval = DateInterval::createFromDateString('-1 week');

Interestingly enough, that does not create a 7 day interval with invert=1. If you var_dump($dateInterval), you'll see public 'd' => int -7 and public 'invert' => int 0.

But technically you don't need DateInterval or DatePeriod to accomplish this.

for ($i=0, $date = new DateTime; $i < 4; $i++) {
    echo $date->modify('last sunday')->format('F j, Y');
}

Thank you for your answer; I agree that this is the best alternative. I really wanted to figure out how to properly get the negative intervals to work. — Talk Nerdy To Me, Feb 16 '18 at 19:41
@TalkNerdyToMe I was only able to figure out one way to make it work with a negative interval, but it's not pretty IMO. — Don't Panic, Feb 16 '18 at 20:20
The invert property is clearly poorly documented. Thank you for your quite comprehensive answer! — Talk Nerdy To Me, Feb 16 '18 at 20:32

Decreasing DateInterval not producing result

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