Código HTML :
<html >
<head>
<title>ejemplo</title>
</head>
<body>
<form name="form1" method="post" action="registra_suma.php">
<table>
<tr><td>numero 1</td><td>numero 2</td><td>total</td></tr>
<tr>
<td><input onKeyUp="if(form1.textfield2.value!=''){form1.textfield3.value = parseInt(this.value)+parseInt(form1.textfield2.value)}" name="textfield1" type="text" size="8" maxlength="8"></td>
<td> <input onKeyUp="if(form1.textfield2.value!=''){form1.textfield3.value = parseInt(this.value)+parseInt(form1.textfield1.value)}" name="textfield2" type="text" size="8" maxlength="8"></td>
<td><input name="textfield3" type="text" size="10" maxlength="10"> </td></tr>
</table>
<p>iva=<?PHP
$iva=0.19;
echo "$iva";?>
</p>
<p>total:</p>
</form>
</body>
</html> 
