Non va.... pagina bianca.....

Deve essere con estensione .php giusto??

codice:
 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento senza titolo</title>

  <script type="text/javascript">
        function Somma(){
        var num1=parseInt(document.form1.txtnum1.value);
        var num2=parseInt(document.form1.txtnum2.value);
        document.form1.txtsomma.value=num1+num2
        }
    </script>


</head>

<body>

  <?


$cn = mysql_connect("localhost", "xxx", "xxx")
or die("Errore: Impossibile connettersi al Database mysql");

mysql_select_db("xxx",$cn)
or die("Errore: Impossibile accedere al database");


$query=SELECT * FROM sma ORDER BY numero DESC LIMIT 1

$ris=mysql_query($query,$cn)
or die("Errore: Impossibile eseguire la query che mi preleva l'ultimo record inserito della dabella sma");

echo "
 
";


$riga=mysql_fetch_array($ris);

$numero=$riga['numero'];


mysql_close();

?>

<form id="form1" name="form1">
    <table border="1" cellspacing="0">
    <tr>
        <td>Num1 che sarebbe il mio "codcliente":</td>
		<td><input type="text" id="txtnum1" name="txtnum1" value="<? echo $numero; ?>" ></td>
    </tr>
    <tr>
        <td>Num2:</td>
        <td><input type="text" id="txtnum2" name="txtnum2" /></td>
    </tr>
    <tr>
        <td>Somma:</td>
        <td><input type="text" id="txtsomma" name="txtsomma" /></td>
    </tr>
    <tr>
    <td colspan="2" align="center"><input type="button" value="somma" onclick="Somma();" /></td>
    </tr>
    
    </table>
</form>
  
  
  
  
  
</p>
</body>
</html>