<form action="procesar.php" method="get">
Primer valor: <input type="text" name="valor1" />
Segundo valor: <input type="text" name="valor2" /><br />
<input type="radio" name="radio" value="sumar" />Sumar<br />
<input type="radio" name="radio" value="restar" />Restar<br />
<input type="radio" name="radio" value="dividir" />Dividir<br />
<input type="submit" name="enviar" value="Enviar">
</form>

<?php
if ($_REQUEST['radio'] == "sumar"){
    $suma = $_REQUEST['valor1'] + $_REQUEST['valor2'];
    echo $suma;
    } elseif ($_REQUEST['radio'] == "restar"){
    $resta = $_REQUEST['valor1'] - $_REQUEST['valor2'];
    echo $resta;
    } elseif ($_REQUEST['radio'] == "dividir"){
        $dividir = $_REQUEST['valor1'] / $_REQUEST['valor2'];
        echo $dividir; }