Final

<!DOCTYPE html>

<?php
    $result = '';
    $sign = (isset($_REQUEST['sign']))? $_REQUEST['sign'] : '?';
    $firstNumber = (isset($_REQUEST['first']))? $_REQUEST['first'] : '';
    $secondNumber = (isset($_REQUEST['second']))? $_REQUEST['second'] : '';


    if (isset($_REQUEST['sign']) && isset($_REQUEST['first']) && isset($_REQUEST['second'])) {
        switch ($_REQUEST['sign']) {
            case "+":
                $result = $_REQUEST['first'] + $_REQUEST['second'];
                break;
            case "-":
                $result = $_REQUEST['first'] - $_REQUEST['second'];
                break;
            case "*":
                $result = $_REQUEST['first'] * $_REQUEST['second'];
                break;
            case "/":
                $result = $_REQUEST['first'] / $_REQUEST['second'];
                break;
        }
    }
?>

<html lang="ru">
<head>
    <meta charset="utf-8" />
    <title>Шаблоны страниц</title>
    <link href="css/style.css" rel="stylesheet" type="text/css" />
</head>
<body>
<div id="main">
    <header>
        <h3>Главная</h3>
        <img class="image" src="img/shapka.jpg" alt="" />

    </header>
        <nav>
            <ul>
                <li><a href="index.php" class="active">Главная</a></li>
                <li><a href="pages/page_1.php">Страница 1</a></li>
                <li><a href="pages/page_2.php">Страница 2</a></li>

            </ul>
        </nav>
               <section>

            <section>
                <article>
                    <header>
                        <h2>
                            Тема 1
                        </h2>
                    </header>
                    <p>
                       text</p>
                </article>
                <article>
                    <header>
                        <h2>
                            Тема 2</h2>
                    </header>
                        <form method = "get" action="index.php">
                                <p><label for="first"></label>
                                    <input type = "text" id = "first" name = "first"
                                           value="<?=$firstNumber?>">
                                    <?=$sign?>
                                    <label for="second"></label>
                                    <input type = "text" id = "second" name = "second"
                                           value="<?=$secondNumber?>">
                                     =
                                    <input type = "text" id = "answer" name = "answer" onkeypress="return false;"
                                           onmousedown="return false;" style="cursor: default"
                                    value="<?=$result?>"> </p>
                                <br/>  <input class="operation" type="submit" name="sign"  value = "+">
                                 <input class="operation" type="submit" name = "sign"  value = "-">
                                 <input class="operation" type="submit" name ="sign"  value = "*">
                                <input class="operation" type="submit" name ="sign"  value = "/">
                       </form>


                </article>

            </section>
            <aside>
                <h2>
                    News</h2>
                <ul>
                    <li><a href="#">New1</a></li>
                    <li><a href="#">New2</a></li>
                    <li><a href="#">New3</a> </li>
                </ul>

            </aside>
        </section>
               <footer>
                   Copyright &copy; 2020 <a href="/" rel="copyright">Localhost</a>.
                   This is the module.
               </footer>
    </div>
</body>
</html>