Age from Estonian idcode

function deciderlab_calc_age_from_idcode($idcode)
{
  if(strlen($idcode) != 11 || !is_numeric($idcode)) {
    return false;
  }
  $century = substr($idcode, 0, 1);
  $year = substr($idcode, 1, 2);
  $month = intval(substr($idcode, 3, 2));
  $day = intval(substr($idcode, 5, 2));
  $centuryPrefix = 0;
  switch($century) {
    case 1:
    case 2:
      $centuryPrefix = '18';
      break;
    case 3:
    case 4:
      $centuryPrefix = '19';
      break;
    default:
      $centuryPrefix = '20';
      break;
  }
  $fullYear = $centuryPrefix.$year;
  $bday = mktime(0, 0, 0, $month, $day, $fullYear);
  $age = intval(substr(date('Ymd') - date('Ymd', $bday), 0, -4));
  return array('age' => $age);
}