weeknummers -Datum uit weeknummers topic 07/01/2006-

<?php
function ISOdates_from_weeknr ($weeknr, $year, $format = '%d-%m-%Y') {
  $start = strtotime ('1 january ' . $year);
  $week1 = strftime ('%V', $start);
  while ($week1 != '01') {
    $start += (7 * 24 * 3600);
    $week1 = strftime ('%V', $start);
  }
  $timestamp_week = strtotime ('+' . ($weeknr - 1) . ' week', $start);
  $weekdag = strftime ('%u', $timestamp_week);
  $dagen_tot_begin_week = $weekdag - 1;
  $dagen_tot_eind_week = 7 - $weekdag;
  $checkdatum = strtotime ("-" . $dagen_tot_begin_week . " days + 3 days", $timestamp_week);
  $check = strftime ('%V', $checkdatum);
  if ($check == '01' && $weeknr > 52) {
    echo 'ongeldig weeknummer voor dit jaar' . "\n";
    return false;
  }
  $begindatum = strtotime ("-" . $dagen_tot_begin_week . " days", $timestamp_week);
  $einddatum = strtotime ("+" . $dagen_tot_eind_week . " days", $timestamp_week);
  return array (strftime ($format, $begindatum), strftime ($format, $einddatum));    
}

    print_r (ISOdates_from_weeknr (1, 2016)); 
?>