Get the first week day in a given week number in PHP - 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 Y-m-d 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