How can I figure out the number of week days in a month?

Question

I'm having this problem right now: given a month and a year, I'd need to know how many week days it has (that is, the number of days excluding Saturday and Sunday).

It seems so simple, and yet I'm baffled. Of course I could solve it with a for loop and check if the day's a Saturday or a Sunday, and if not increment a counter, but this is just plain stupid (and linear time) considering I'm pretty sure I could get away with a couple of divisions or modulos.

Any idea of the algorithm? You have all the power of PHP 4.4.1 at your disposal.

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EDIT Here's a working for loop implementation:

function weekdays_in_month($month, $year)
{
    $days_in_month = days_in_month($month); // days_in_month defined somewhere
    $first_day = date('w', mktime(0,0,0, $month, 1, $year));
    $counter = 0;
    for ($i = 0; $i < $days_in_month; $i++)
    {
        if (($first_day + $i + 1) % 7 >= 2)
            $counter++;
    }
    return $counter;
}

Answer 1

Just check the weekday-ness of the 29th, 30th, and 31st (if these dates exist).

Add 20.

Editing your function:

function weekdays_in_month($month, $year)
{
    // NOTE: days_in_month needs $year as input also, to account for leap years
    $days_in_month = days_in_month($month, $year); // days_in_month defined somewhere
    $first_day = date('w', mktime(0,0,0, $month, 1, $year));
    $counter = 20;  // first 28 days of month always have 20 weekdays
    for ($i = 28; $i < $days_in_month; $i++)
    {
        if (($first_day + $i + 1) % 7 >= 2)
            $counter++;
    }
    return $counter;
}