How to Calculate the Days of a Week in PHP Given a Week Number or a Specific Date?

Calculating Days of a Week

When given a week number, such as obtained from date -u %W, we might need to calculate the days within that week, starting from Monday. This allows us to generate a specific list of dates within that week.

PHP Implementation

In PHP, we can utilize a loop to generate the dates:

<code class="php">$week_number = 40;
$year = 2008;
for ($day = 1; $day <= 7; $day++) {
    echo date('m/d/Y', strtotime($year . "W" . $week_number . $day)) . "\n";
}

Output:

10/06/2008
10/07/2008
10/08/2008
10/09/2008
10/10/2008
10/11/2008
10/12/2008

Obtaining Dates from a Given Date

An alternative approach is to obtain a specific week's dates given a specific day. For instance, to get the dates of the week containing October 7th, 2008:

<code class="php">function week_from_monday($date) {
    // Assuming $date is in format DD-MM-YYYY
    list($day, $month, $year) = explode("-", $date);

    // Get the weekday of the given date
    $wkday = date('l', mktime(0, 0, 0, $month, $day, $year));

    switch ($wkday) {
        case 'Monday': $numDaysToMon = 0; break;
        case 'Tuesday': $numDaysToMon = 1; break;
        case 'Wednesday': $numDaysToMon = 2; break;
        case 'Thursday': $numDaysToMon = 3; break;
        case 'Friday': $numDaysToMon = 4; break;
        case 'Saturday': $numDaysToMon = 5; break;
        case 'Sunday': $numDaysToMon = 6; break;
    }

    // Timestamp of the Monday for that week
    $monday = mktime(0, 0, 0, $month, $day - $numDaysToMon, $year);

    $seconds_in_a_day = 86400;

    // Get date for 7 days from Monday (inclusive)
    for ($i = 0; $i < 7; $i++) {
        $dates[$i] = date('Y-m-d', $monday + ($seconds_in_a_day * $i));
    }

    return $dates;
}

Output:

Array
(
    [0] => 2008-10-06
    [1] => 2008-10-07
    [2] => 2008-10-08
    [3] => 2008-10-09
    [4] => 2008-10-10
    [5] => 2008-10-11
    [6] => 2008-10-12
)</code>

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