Get year and week number as 6 digit integer from any DateTime object including fix for in between the years

getCalendarYearWeek.php

<?php

/**
 * @param \DateTime $dateTime
 *
 * @return int - e.g. 202001
 */
protected function getCalendarYearWeek(DateTime $dateTime): int
{
    $week = $dateTime->format('W');

    // the calendar week starts on monday
    $dateTime->modify('monday this week');

    // on first week (1) and last week (52 or 53) of the year monday and sunday can be in different years.
    // the actual year of the current calendar week is depending on which year has more days in this week.
    if ($week == $this->getLastCalendarWeek($dateTime) || $week == 1) {
        $checkDateTime = clone $dateTime;
        // if thursday is in the new year (1/2/3/4)
        // we switch the day to sunday to set the new year
        if ($checkDateTime->modify('thursday this week')->format('d') <= 4) {
            $dateTime->modify('sunday this week');
        }
    }

    return (int)$dateTime->format('Y') . sprintf('%02d', $week);
}

/**
 * @param \DateTime $dateTime
 *
 * @return int
 */
protected function getLastCalendarWeek(DateTime $dateTime): int
{
    $dateTime = clone $dateTime;

    $dateTime->setISODate($dateTime->format('Y'), 53);

    return ($dateTime->format('W') === '53' ? 53 : 52);
}

I was thinking about a solution for the problem that depending on the weekday the datetime object can return different years for the same calendar week. Whether the current calendar week counts into the old or the new year depends on the first thursday of the year, see: https://en.wikipedia.org/wiki/ISO_week_date#First_week

Maybe I overcomplicated things here while trying to solve the puzzle. If you can think about a more elegant solution please let me know!