online Thai ssn generator and verifier

<form method="post" action="">
<input type="submit" value="Generate" name="Generate">
</form>
<?
if(!isset($ssnin)){
    $letter[0]        = rand(1,8);
    for($a=1;$a<12;$a++){
        $letter[$a]        = rand(0,9);
    }
    for($i=0;$i<13;$i++){
        $multi[$i] = (13-$i) * $letter[$i];
    }
    $summ = array_sum($multi);
    $c = $summ%11;
    if($c>2){$d = 11-$c;}else{$d=$c;}
    $letter[12] = $d;
    $tssn = $letter[0].$letter[1].$letter[2].$letter[3].$letter[4].$letter[5].$letter[6].$letter[7].$letter[8].$letter[9].$letter[10].$letter[11].$letter[12];
    print("Generated SSN for Thai is : ");
    print("<font color=red>".$tssn."</font>");
}
?>
<HR>
<!-- Verify section -->
<center>VERIFIED SSN?</center><BR>
<form method="post" action="">
SSN : <input type="text" size="13" name="ssnin" value="<?print($tssn)?>">
<input type="submit" value="Verified" name="submit">
</form>
<?
if(isset($ssnin)){
    $summary = 0;
    for($i=0;$i<12;$i++){
        $summary = $summary + $ssnin[$i]*(13-$i);
    }
    $summary = $summary%11;
    if($summary >2){$dumm = 11-$summary ;}else{$dumm=$summary ;}
    if($dumm == $ssnin[12]){
        print("SSN : ".$ssnin." is VALID");
    }else if($dumm != $ssnin[12]){
        print("SSN : ".$ssnin." is NOT VALID......last number must be ".$dumm );
    }
}
?>