// determine days number to next monday
$daysToNextMo = ($defaultWeekDay == 0) ? 1 : (8 - $defaultWeekDay);

// determine days number to prev sunday
$daysToPrevSu = ($defaultWeekDay == 0) ? 7 : $defaultWeekDay;

// determine start date of next week
// next monday day is out of days in defaultMonth
if (($defaultDay + $daysToNextMo) > date('t', $defaultTimestamp)) {
  $nextDay = $daysToNextMo - (date('t', $defaultTimestamp) - $defaultDay);
  if ($defaultMonth == 12) {
    $nextMonth = 1;
    $nextYear = $defaultYear + 1;
  }
  else {
    $nextMonth = $defaultMonth + 1;
    $nextYear = $defaultYear;
  }
}
else {
  $nextDay = $defaultDay + $daysToNextMo;
  $nextMonth = $defaultMonth;
  $nextYear = $defaultYear;
}
$nextDate = $nextYear.'-'.$nextMonth.'-'.$nextDay;