how to find the last monday of the month - php

Question

I'm not sure how to go about this one. I'm building a calendar in PHP and need users to be able to add a repeating event that follows the following rule:
Last [DOW] of the month (so Last [Mon/Tues/Wed/etc] of the week)
I've got the rule itself stored, I'm just not sure how best to extrapolate the last Mon/Tue/Wed of the month in PHP? I fell like i'm making this more complicated than it needs to be.
Assuming you have variables for $month=4, $dow=3 and $year=2011 how would I best do this?

Answer 1

For everything date-related that you can express in proper English but have a hard time expressing using numbers, strtotime is your best friend.
echo strtotime("last Monday of June 2011");
This returns a timestamp that you can use as the second parameter to date and the likes to get a proper, human-readable date. Since it's a built-in function written in C, this solution is also much faster than almost anything else you could come up with written in PHP (though I'm quite sure it wouldn't matter much in a real-world scenario).
So assuming you have $month=4, $dow=3 and $year=2011, you'll need an array mapping $month values to their English textual representations and another array mapping $dow values to their textual representations.

Answer 2

Here's an alternative:
<?
function lastDayOfMonth($month, $year) {
switch ($month) {
case 2:
# if year is divisible by 4 and not divisible by 100
if (($year % 4 == 0) && ($year % 100) > 0)
return 29;
# or if year is divisible by 400
if ($year % 400 == 0)
return 29;
return 28;
case 4:
case 6:
case 9:
case 11:
return 30;
default:
return 31;
}
}
function lastDayOfWeek($month, $year, $dow) {
$d = new DateTime();
#Finding the last day of month
$d = $d->setDate($year, $month, lastDayOfMonth($month, $year));
#Getting the day of week of last day of month
$date_parts = getdate($d->getTimestamp());
$diff = 0;
#if we can't find the $dow in this week... (cause it would lie on next month)
if ($dow > $date_parts['wday']) {
# ...we go back a week.
$diff -= 7;
}
return $date_parts['mday'] + $diff + ($dow - $date_parts['wday']);
}
# checking the algorithm for this month...
for ($i=0; $i < 7; $i++) {
echo lastDayOfWeek(6,2011,$i) . "<br>";
}
?>

Answer 3

I have a generic function for you to calculate the nth day of a month. Hope this could help you to resolve your issue.
function get_Nth_dow($dow, $occurence, $m, $y)
{
$numdays = date('t', mktime(0, 0, 0, $m, 1, $y));
$add = 7 * ($occurence - 1);
$firstdow = date('w', mktime(0, 0, 0, $m, 1, $y));
$diff = $firstdow - $dow;
$day_of_month = 1;
if ($diff > 0)
{
$day_of_month += ($add - $diff);
}
elseif ($diff < $numdays)
{
$day_of_month -= ($diff - $add);
}
return $day_of_month;
}
$DOW = day of the week (0 = Sunday, 6 = Saturday).
$X = occurence (1 = first, 2 = third, etc..). If the given month does
not have the occurrence, then it will return the last. For example, if
you ask for the 7th occurrence of Friday, it will return the last
Friday of the month.
$M = month
$Y = year
Example, get_Nth_DOW(2,3,7,2009) will return the third Tuesday of 7th 2009.