Get the first week day in a given week number in PHP

Question

How can I get the first week day in a given week number? I'm making a function in PHP for a calendar app.

The idea: When I click on a link that basically uses strtotime with +1 month it only jumps to that same day of course. I need to get the week numbers correct.

Example: When I use the menu to move from August to September, it shouldn't select the same date I was in in August, but the first day of the first week number in September (Monday, 2th of September, week number 36).

And from September to October: Week number 40, Tuesday the 1th of October.

Answer 1

I found a function that exactly like you want it. This is setISODate

$date = new DateTime();
$date->setISODate(2013, 35, 1);
echo $date->format('Y-m-d');

You can change Ymd date format as you want

Output

2013-08-26 // This week number and monday

Usage

setISODate(year, week, day)

Answer 2

Try this code

<?php
        $week = 3;
        $year = 2009;

        $timestamp = mktime( 0, 0, 0, 1, 1,  $year ) + ( $week * 7 * 24 * 60 * 60 );
        $timestamp_for_monday = $timestamp - 86400 * ( date( 'N', $timestamp ) - 1 );
        $date_for_monday = date( 'Y-m-d', $timestamp_for_monday );
?>

Answer 3

Like every1 already said, ISO weeks start with monday. Quote from http://en.wikipedia.org/wiki/ISO_week_date : Weeks start with Monday.

If I understand your problem correctly, you need first-next-date in next month if selected week is in 2 different months?

function getFirstWeekDay($year, $week) {
    $dt = (new DateTime)->setISODate($year, $week, 1);
    $dtC = clone $dt;
    for ($N = $dt->format('N'); $N < 7; $N++) {
        $dtC->modify('+1 day');
        if ($dt->format('m') !== $dtC->format('m')) {
            return $dtC;
        }
    }
    return $dt;
}

echo getFirstWeekDay(2013, 40)->format('W\t\h \w\e\e\k \i\s Y-m-d'); 
# 40th week is 2013-10-01

echo getFirstWeekDay(2013, 1)->format('W\t\h \w\e\e\k \i\s Y-m-d'); 
# 1th week is 2013-01-01