i live in sweden, where displaying weeknumbers in calendars of any sort is common. the
following code does the work, tho i'd be very happy if it could be supported to be returned
from the date() function.

switch(date("w",mktime(0,0,0,$mon,$daycnt+1,$year))) {
	case 1: $daysub=0; break;
	case 2: $daysub=1; break;
	case 3: $daysub=2; break;
	case 4: $daysub=3; break;
	case 5: $daysub=4; break;
	case 6: $daysub=5; break;
	case 0: $daysub=6; break;
}
$weeknumber=round((date("z", mktime(0,0,0, date("m"),date("d")+1-$daysub,date("Y")))+7)/7);
if($weeknumber==53) {
	//some years have 53 weeks & some 52
	//so we check next weeks weeknumber, if its 1 this is week 53, otherwise, this is week 1.
	switch(date("w",mktime(0,0,0,date("m"),date("d")+8,date("Y")))) {
		case 1: $daysub=0; break;
		case 2: $daysub=1; break;
		case 3: $daysub=2; break;
		case 4: $daysub=3; break;
		case 5: $daysub=4; break;
		case 6: $daysub=5; break;
		case 0: $daysub=6; break;
	}
	$tmpweeknumber=round( (date("z", mktime(0,0,0, date("m"), date("d")+8-$daysub, date("Y")))+7)/7);
	if($tmpweeknumber==1) $weeknumber=53;
	else $weeknumber=1;
}

You can use the strftime function already, it's the %W modifier.
But it would be useful if date would support it indeed.

Not closed...

Trouble is (I suppose), some week-of-year functions consider Monday the first day of the week, and some consider Sunday.

I guess two switches in date() should be done.  I'll work on this.

date('W') has been added to CVS.

It returns the ISO 8601 week number (0-53).  Basically, Monday is the first day of the week, and week #1 of the year is the week that contains January 4.

- Colin