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

Calculating Days of the Week Given a Week Number

Given a week number, such as the one obtained using the date -u %W command, it is often useful to generate the corresponding dates for the days of that week. Let's start with an example.

Consider week number 40 of the year 2008. Using ISO 8601's rules for week numbering, we expect the days in that week to be:

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

PHP Solution:

In PHP, the date function provides various formatting options. Here's a simple loop that prints the dates for the days of a given week number:

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

This loop iterates through the 7 days of the week and prints the corresponding dates in the format: YYYY-MM-DD.

PHP Solution for Calculating Days of a Week from a Given Date:

A different scenario involves calculating the days of a week given a specific date. Here's a PHP function that does this:

<code class="php">function week_from_monday($date) {
    // Extract date parts
    list($day, $month, $year) = explode("-", $date);

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

    // Calculate the number of days to subtract to get Monday
    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;   
    }

    // Get timestamp of Monday
    $monday = mktime('0','0','0', $month, $day-$numDaysToMon, $year);

    // Calculate dates for 7 days from Monday
    $dates = array();
    $seconds_in_a_day = 86400;
    for($i=0; $i<7; $i++)
    {
        $dates[$i] = date('Y-m-d',$monday+($seconds_in_a_day*$i));
    }

    return $dates;
}

Example usage:

<code class="php">$dates = week_from_monday('07-10-2008');
print_r($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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