<?php
$a=0; $b=0; $c; $res=0; // variables
if (isset ($_POST ['calc '])) { /* check if the submit button was clicked */
$a = $_POST['num1 ']; //writing the first value to a variable a
$b = $_POST['num2 ']; //writing the second value to a variable b
$c = $_POST['sign ']; //writing the arithmetic sign to a variable с
switch ($c) {
case "+": $res = $a + $b; // performing an action
echo "<INPUT type=text value=\"$res\">"; // output the result
break;

// ... code for calculating subtraction, multiplication, and division
default: echo "<INPUT type=text value=\"'Не указано действие'\">";
}
} ?>
</body>
</html>