Hola de nuevo.
Prueba este código:
Código PHP:
<html>
<head>
<script type="text/javascript">
function sumar(frm) {
s1 = frm.txt1.value;
s2 = frm.txt2.value;
if (s1=='' || isNaN(s1)) s1 = 0;
if (s2=='' || isNaN(s2)) s2 = 0;
frm.total.value = parseFloat(s1) + parseFloat(s2);
}
</script>
</head>
<body>
<form>
<input type="text" name="txt1" onkeyup = "sumar(this.form)" />
<input type="text" name="txt2" onkeyup = "sumar(this.form)" />
<input type="text" name="total" readonly = "readonly" />
</form>
</body>
</html>
Saludos,
