Código PHP:
<form name="form1" action="" method="post" >
<p>
<input type="text" name="textfield">
+
<input type="text" name="textfield2">
</p>
<p>
<input type="submit" name="Submit" value="Enviar">
</p>
<?
if($_POST['Submit']){
$textfield = $_POST['textfield'];
$textfield2 = $_POST['textfield2'];
$total = $textfield + $textfield2;
echo $total;
}
?>