/**
 * date('W') liefert für uns die falsche Wochennummer. Wir wollen nach ISO 8601:1988
 * (erste Woche = Woche mit mindestens 4 Tage des neuen Jahres
 * @param timestamp $date
 * @return int
 */
function getWeekNr($date){
	//Windows kann nicht alle Formatierbefehle. Darum muss die Woche nach ISO 8601:1988 Format anderst hergeholt werden
	//Version für Windows:
	if (stristr(PHP_OS, "win")<>false){
		return getWeekNumber(date('j', $date), date('n', $date), date('Y', $date));
	} else {
	//Version für Unix:
		return (Integer) strftime('%V', $date);
	}
}

// [url]http://theserverpages.com/php/manual/en/function.strftime.php[/url]
// Get the week number in ISO 8601:1988 format
// This helsp when %V is not working on your Win32 machine
function getWeekNumber($day, $month, $year) {
 $week = strftime("%W", mktime(0, 0, 0, $month, $day, $year));
 $yearBeginWeekDay = strftime("%w", mktime(0, 0, 0, 1, 1, $year));
 $yearEndWeekDay  = strftime("%w", mktime(0, 0, 0, 12, 31, $year));
 // make the checks for the year beginning
 if($yearBeginWeekDay > 0 && $yearBeginWeekDay < 5) {
  // First week of the year begins during Monday-Thursday.
  // Currently first week is 0, so all weeks should be incremented by one
  $week++;
 } else if($week == 0) {
  // First week of the year begins during Friday-Sunday.
  // First week should be 53, and other weeks should remain as they are
  $week = 53;
 }
 // make the checks for the year end, these only apply to the weak 53
 if($week == 53 && $yearEndWeekDay > 0 && $yearEndWeekDay < 5) {
  // Currently the last week of the year is week 53.
  // Last week of the year begins during Friday-Sunday
  // Last week should be week 1
  $week = 1;
 }
 // return the correct ISO 8601:1988 week (in two digit format)
 return( substr('0'. $week, -2) );
}