DanielKassner.com

<?php
function GetDayByPosition($Position, $Weekday, $Month, $Year) {
	// Sanatize some of the inputs
	$Position = strtolower($Position);
	$Weekday = strtolower($Weekday);
	
	// Go to next month so we can then go back 1 week at end of script for calculating last xxx of month
	if ($Position=='last') { $Month += 1; }
        // We cannot have 13 months so set as January of the next year.
	// This was pointed out by Evan who made a comment on the posting of this script
	if ($Month > 12) {
		$Month = 1;
		$Year += 1;
	}
	// Create new date object for the first of the month
	$D = new DateTime($Year.'-'.$Month.'-1');
	$DOW = $D->format('w'); // Get the current day of the week based on the 1st of the month
	$keys = array('sunday'=>0,'monday'=>1,'tuesday'=>2,'wednesday'=>3,
				  'thursday'=>4,'friday'=>5,'saturday'=>6);
	// Calculate what the offset is based on the current day of the week
	// vs the day in the week you want to get
	$offset = $keys[$Weekday] - $DOW;
	if ($offset<0) { $offset += 7; }

	switch ($Position) {
		// Don't need to add anything to first
		case 'second':
			$offset += 7; // Add 1 week
			break;
		case 'third':
			$offset += 14; // Add 2 weeks
			break;
		case 'fourth':
			$offset += 21; // Add 3 weeks
			break;
		case 'last':
			$offset -= 7; // Go back 7 days
			break;
	}
	// Add the current offset of days to the date object
	$D->modify('+'.$offset.' days');
	return $D->format('Y-m-d');
	
}
?>

<?php
// This will output 2009-01-05
echo 'First Monday of January 2009: ',GetDayByPosition('first', 'monday', 1, 2009),'<br />';
// This will output 2009-01-13
echo 'Second Tuesday of January 2009: ',GetDayByPosition('second', 'tuesday', 1, 2009),'<br />';
// This will output 2009-01-21
echo 'Third Wednesday of January 2009: ',GetDayByPosition('third', 'wednesday', 1, 2009),'<br />';
// This will output 2009-01-22
echo 'Fourth Thursday of January 2009: ',GetDayByPosition('fourth', 'thursday', 1, 2009),'<br />';
// This will output 2009-01-30
echo 'Last Friday of January 2009: ',GetDayByPosition('last', 'friday', 1, 2009),'<br />';
?>