Código :
<html>
<head>
<script language="javascript">
function sumar()
{
var a, total = 0;
var elements = document.getElementsById('sumar');
for(a=0; a<elements.length; a++){
total += (parseFloat(elements[a].value));
}
document.getElementById("resultado").value = total;
}
</script>
<title>suma y suma</title>
</head>
<body>
<?php
$v = 1;
if(isset($_POST["ok"]))
{
$numero = $_POST["valor"];
$numero++;
$v = $numero;
}
?>
<form method="post" action="#">
<?php for($a=0;$a<$v;$a++) { ?>
<input type="text" name="<?php echo "sumar$a"; ?>" <?php if(isset($_POST["ok"])) echo "value='" . $_POST["sumar$a"] . "'"; ?> id='sumar' onKeyUp="sumar()" size="8" />+
<?php } ?>
//
<input type="hidden" name="valor" value="<?php echo $a; ?>" />
<input type="submit" name="ok" value="otro input"/>=
<input type="text" name="total" id="resultado" readonly="readonly" />
</form>
</body></html>
