Calculate your b'day from ID number with php

<?php
if(isset($_REQUEST['id_no'])){
 $id_no=$_REQUEST['id_no'];
}
else
 $id_no="";
?>

<form name="idcalc_form" method="get" action="">
<input type="text" name="id_no" value="<?php echo $id_no;?>" pattern="[0-9]{9}[V]" autofocus placeholder="NIC number"/>
<input type="submit" name="idcalc">
</form>
<?php

if(isset($_REQUEST['id_no']) && isset($_REQUEST['idcalc'])){
 $year=substr($id_no,0,2);
 $days=substr($id_no,2,3);

 function checkNum($between) {
   if($between=='' || $between=='000' || ($between > 366 && $between < 501) || ($between > 866)) {
  throw new Exception("Value must be between 1 - 366 and 501 - 866 (inclusive)");
   }
   return true;
 } 
 try {
   checkNum($days);
  if($days<=366){$gender='Male';}else{$gender='Female'; $days=$days-500;}

  if($days>='001' && $days<='031'){
   $month=array('Jan',1);
  }elseif($days>='032' && $days<='060'){
   $month=array('Feb',32);
  }elseif($days>='061' && $days<='091'){
   $month=array('Mar',61);
  }elseif($days>='092' && $days<=121){
   $month=array('Apr',92);
  }elseif($days>=122 && $days<=152){
   $month=array('May',122);
  }elseif($days>=153 && $days<=182){
   $month=array('Jun',153);
  }elseif($days>=183 && $days<=213){
   $month=array('Jul',183);
  }elseif($days>=214 && $days<=244){
   $month=array('Aug',214);
  }elseif($days>=245 && $days<=274){
   $month=array('Sep',245);
  }elseif($days>=275 && $days<=305){
   $month=array('Oct',275);
  }elseif($days>=306 && $days<=335){
   $month=array('Nov',306);
  }elseif($days>=336 && $days<=366){
   $month=array('Dec',336);
  }
  else $month=array('bogus',0);
  $date=$days-$month[1]+1;
  echo $year.' '.$month[0].' '.$date.' '.$gender;
 }
 catch(Exception $e) {
   echo 'Error: ' .$e->getMessage();
 }
}
?>