PHP - get last week number in year

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

Solution 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.

Solution 2:

In ISO-8601 specification, it says that December 28th is always in the last week of its year.
Based on that, we can simply create that date, and see in what week it is:

$dt = new DateTime('December 28th');
echo $dt->format('W'); # 52

$dt = new DateTime('December 28th, 2009');
echo $dt->format('W'); # 53

demo

^{... or if you are using date() and strtotime() functions: echo date('W', strtotime('December 28th')); # 52}