PHP - Demo Operator Aritmatika

<?php
    // mengecek pengiriman form
    if (isset($_POST['opr'])) {
        // echo "<pre>"; print_r($_POST); echo "</pre>";
        extract($_POST);
        if (is_numeric($bil1) && is_numeric($bil2)) {
            $hasil =  $bil1." ".$opr." ".$bil2. " = ";
            switch($opr) {
                case "+": $hasil .= $bil1+$bil2; break;
                case "-": $hasil .= $bil1-$bil2; break;
                case "*": $hasil .= $bil1*$bil2; break;
                case "/": $hasil .= $bil1/$bil2; break;
                case "%": $hasil .= $bil1%$bil2; break;
                case "**": $hasil .= $bil1**$bil2; break;
                default: echo 'Operator salah!';
            }
        } else {
            $error = "Operand yang diinputkan bukan bilangan!";
        }
    }
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-QWTKZyjpPEjISv5WaRU9OFeRpok6YctnYmDr5pNlyT2bRjXh0JMhjY6hW+ALEwIH" crossorigin="anonymous">
    <title>Document</title>
</head>
<body>
    <form action="<?= $_SERVER['PHP_SELF'] ?>" method="post">
        <!-- <label for="bil1">Bilangan 1</label> -->
        <input placeholder="bilangan 1" type="text" size="8" name="bil1">
        <select name="opr">
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
            <option value="%">%</option>
            <option value="**">**</option>
        </select>
        <!-- <label for="bil2">Bilangan 2</label> -->
        <input placeholder="bilangan 2" type="text" size="8" name="bil2">
        <input type="submit" value="Hitung">
    </form>
    <?php if (isset($hasil)) { ?>
        <div class="alert alert-success" role="alert">
            <?= $hasil; ?>
        </div>
    <?php } ?>
    <?php if (isset($error)) { ?>
        <div class="alert alert-danger" role="alert">
            <?= $error; ?>
        </div>
    <?php } ?>
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/js/bootstrap.bundle.min.js" integrity="sha384-YvpcrYf0tY3lHB60NNkmXc5s9fDVZLESaAA55NDzOxhy9GkcIdslK1eN7N6jIeHz" crossorigin="anonymous"></script>
</body>
</html>