PHP - get last week number in year

Question

Tell me please how to get the last number of weeks in a year?

Answer 1

function getIsoWeeksInYear($year) {
    $date = new DateTime;
    $date->setISODate($year, 53);
    return ($date->format("W") === "53" ? 53 : 52);
}

The o date format gives the ISO-8601 year number. We can use this, and the fact that "invalid" dates are automatically rolled around to make valid ones (2011-02-31 gives 2011-03-03), to determine if a given year has 53 weeks. If does not, then it must have 52.

See also, date format characters and DateTime::setISODate() manual pages.