<?php
/**
 * Created by PhpStorm.
 * User: Maks
 * Date: 25.05.2016
 * Time: 8:00
 */

session_start();

$first_argument = $_POST["first_argument"];
$second_argument = $_POST["second_argument"];
$result = 0;
$action = "";

if (isset($_POST["plus"])) {
    $action = "+";
    $result = $first_argument + $second_argument;
}
elseif (isset($_POST["minus"])) {
    $action = "-";
    $result = $first_argument - $second_argument;
}
elseif (isset($_POST["multiply"])) {
    $action = "*";
    $result = $first_argument * $second_argument;
}
elseif (isset($_POST["divide"])) {
    $action = "/";
    $result = $first_argument / $second_argument;
}

$_SESSION["result"] = $result;
?>

<form action="index2.php" method="post" name="calculator">
    <input type = "text" name="first_argument" style="width:100px;">
    <?php echo $action ?>
    <input type = "text" name="second_argument" style="width:100px;">
    =
    <?php echo $result; ?>
    <br>
    <input type="submit" value="+" name="plus">
    <input type="submit" value="-" name="minus">
    <input type="submit" value="*" name="multiply">
    <input type="submit" value="/" name="divide">
</form>