$predefinedYear = 2009;
$predefinedWeeks = 47;

// find first mоnday of the year
$firstMon = strtotime("mon jan {$predefinedYear}");

// calculate how much weeks to add
$weeksOffset = $predefinedWeeks - date('W', $firstMon);

// calculate searched monday
$searchedMon = strtotime("+{$weeksOffset} week " . date('Y-m-d', $firstMon));

//To calculate 12 th Monday from this Monday(2014-04-07)
    $n_monday=12;
    $cur_mon=strtotime("next Monday");
    for($i=1;$i<=$n_monday;$i++){
       echo date('Y-m-d', $cur_mon);
       $cur_mon=strtotime(date('Y-m-d', strtotime("next Monday",$cur_mon)));
    }

2014-04-07
2014-04-14
2014-04-21
2014-04-28
2014-05-05
2014-05-12
2014-05-19
2014-05-26
2014-06-02
2014-06-09
2014-06-16
2014-06-23

function getMondaysDate($year, $week) {
  if (!is_numeric($year) || !is_numeric($week)) {
    return null;
    // or throw Exception, etc.
  }

  $timestamp = strtotime("+$week weeks Monday January $year");
  $prettyDate = date('d M Y');
  return $prettyDate;
}

date('c',strtotime('Sunday Jan 2018'));
// "2018-01-07T00:00:00+11:00" (or whatever your timezone is)

date('c',strtotime('+1 weeks Sunday Jan 2018'));
// "2018-01-14T00:00:00+11:00" (or whatever your timezone is)

date('c',strtotime('+52 weeks Sunday Jan 2018'));
// "2019-01-06T00:00:00+11:00"