<?
/* weeknumber.php3
 
   (int) get_week_number($timestamp)
 
   Week number of year with Monday as first day of the week
   (1...53). If the week containing January 1 has four or more days
   in the new year, then it is considered week 1; otherwise, it is
   week 53 of the previous year, and the next week is week 1. (See the
   ISO 8601: 1988 standard.)
 
   Adapted from GNU sh-utils for PHP3 by Stefan Röhrich.
 
*/
 
function is_leap_year($year) {
        if ((($year % 4) == 0 and ($year % 100)!=0) or ($year % 400)==0) {
                return 1;
        } else {
                return 0;
        }
}
 
/*
#define ISO_WEEK1_WDAY 4 // Thursday
  int big_enough_multiple_of_7 = (-YDAY_MINIMUM / 7 + 2) * 7;
  return (yday
          - (yday - wday + ISO_WEEK1_WDAY + big_enough_multiple_of_7) % 7
          + ISO_WEEK1_WDAY - ISO_WEEK_START_WDAY);
*/
 
function iso_week_days($yday, $wday) {
        return $yday - (($yday - $wday + 382) % 7) + 3;
}
 
function get_week_number($timestamp) {
 
        $d = getdate($timestamp);
 
        $days = iso_week_days($d["yday"], $d["wday"]);
 
        if ($days < 0) {
                $d["yday"] += 365 + is_leap_year(--$d["year"]);
                $days = iso_week_days($d["yday"], $d["wday"]);
        } else {
                $d["yday"] -= 365 + is_leap_year($d["year"]);
                $d2 = iso_week_days($d["yday"], $d["wday"]);
                if (0 <= $d2) {
                        /* $d["year"]++; */
                        $days = $d2;
                }
        }
 
        return (int)($days / 7) + 1;
}
 
echo get_week_number(time());
?>