<?php

ini_set('display_errors', 1);

$result = '';

if(isset($_POST['calculate'])) {

    if(empty($_POST['firstValue']) && empty($_POST['lastValue'])) { // here $_POST
             echo  "<script>alert('Please enter values')</script>";
        }

        elseif (empty($_POST['firstValue'])) {// here $_POST
             echo  "<script>alert('Please enter first value')</script>";
        }
        elseif (empty($_POST['lastValue'])) {// here $_POST
             echo  "<script>alert('Please enter second value')</script>";
        }
        elseif (filter_var($_POST['firstValue'], FILTER_VALIDATE_INT) === false) {// here $_POST
             echo  "<script>alert('First value must be numeric')</script>";
        }
        elseif (filter_var($_POST['lastValue'], FILTER_VALIDATE_INT) === false) {// here $_POST
             echo  "<script>alert('Last value must be numeric')</script>";
        }

        else {

                    $operator = $_POST['operator'];

                    switch ($operator) {
                            case '+':
                                            $add1 = $_POST['firstValue'];
                                            $add2 = $_POST['lastValue'];
                                            $result = $add1 + $add2;
                                            break; // here

                            case '-':
                                            $add1 = $_POST['firstValue'];
                                            $add2 = $_POST['lastValue'];
                                            $result = $add1 - $add2;
                                            break; // here
                            case '*':
                                            $add1 = $_POST['firstValue'];
                                            $add2 = $_POST['lastValue'];
                                            $result = $add1 * $add2;
                                            break; // here
                            case '/':
                                            $add1 = $_POST['firstValue'];
                                            $add2 = $_POST['lastValue'];
                                            $result = $add1 / $add2;
                                            break;

                            default:
                                            $result = '';
                                            echo "not accepted";
                                            break;
                    }

        }

}


?>
<form action="" method="POST"> <!-- here method="POST" -->
    <table style="border: groove #00FF99">
        <tr>

        <td style="background-color: aqua; color: red; font-family: Arial;">Enter first number</td>
        <td colspan="1"><input type="text" name="firstValue" style="color: red;"></td>

        <tr>
            <td style="color:burlywood; font-family: Arial;">Select Operator</td>
            <td>
                <select name="operator" style="width: 63px;">
                    <option>+</option>
                    <option>-</option>
                    <option>*</option>
                    <option>/</option>
                </select>
            </td>
        </tr>

        <tr>

        <td style="background-color: aqua; color: red; font-family: Arial;">Enter second number</td>
        <td class="auto-style5"><input type="text" name="lastValue" style="color: red;"></td>

        </tr>

                <tr>
                    <td></td>
                    <td><input type="submit" name="calculate" value="Calculate" style="color: wheat; background-color: rosybrown;">         </td>
                </tr>

                <tr>
                    <td style="background-color: aqua; color: red;">Output =</td>
                    <td style="color: darkblue;"><?php echo $result; ?></td>
                </tr>

                </tr>
            </table>
        </form>






</body>
</html>