Обрабатывайте несколько кнопок отправки с различными действиями в PHP.
<?php
extract($_POST);
//do addition and store the result in $res
if(isset($add))
{
$res=$fnum+$snum;
}
//do subtraction and store the result in $res
if(isset($sub))
{
$res=$fnum-$snum;
}
//do multiplicatio and store the result in $res
if(isset($mult))
{
$res=$fnum*$snum;
}
?>
<html>
<head>
<title>Display the result in 3rd text box</title
>
</head>
<body>
<form method="post">
<table align="center" border="1">
<Tr>
<th>Result</th>
<td><input type="text" value="<?php echo @$res;?>"/></td>
</tr>
<tr>
<th>Enter first number</th>
<td><input type="text" name="fnum"/></td>
</tr>
<tr>
<th>Enter second number</th>
<td><input type="text" name="snum"/></td>
</tr>
<tr>
<td align="center" colspan="2">
<input type="submit" value="+" name="add"/>
<input type="submit" value="-" name="sub"/>
<input type="submit" value="*" name="mult"/>
</tr>
</table>
</form>
</body>
</html>