Simple php in wordpress widget [closed]

add_action( 'wp_dashboard_setup', 'register_my_dashboard_widget' );
function register_my_dashboard_widget() {
    wp_add_dashboard_widget(
        'net_profit_widget',
        'Net Profit Calculator',
        'net_profit_widget_display'
    );

}

function net_profit_widget_display() {

    $subPrice = 0;
    $subnumb = 0;
    if (array_key_exists('subprice', $_POST) !== false) {
        $subPrice = $_POST['subprice'];
    }
    if (array_key_exists('subnumb', $_POST) !== false) {
        $subnumb = $_POST['subnumb'];
    }


    $payperc = "2.9";
    $paystan = "0.3";
    $netprof = "N/A";
    $expense = "N/A";



    if ($_SERVER['REQUEST_METHOD'] == 'POST') {
        if ($subPrice === false || $subnumb === false ) {
            die("Please fill in all the required information!");
        } else {
            $expense = $subPrice * (($payperc / 100) + $paystan);
            $netprof = ($subPrice - $expense) * $subnumb;
        }
    }
?>

    <div style="color: #777;">Paypal fee per transaction: 2.9% + $0.3</div><br>
    <form class="netcalc" method="POST" action="#">
        <label for="sub-price">Subscription Price($)</label><input name="subprice" id="sub-price" type="text" value="<?php echo $subPrice; ?>">
        <br><label for="total-subs">Subscriptions Sold</label><input name="subnumb" id="total-subs" type="text" value="<?php echo $subnumb; ?>" >
        <br><br><div><span class="expenses-label">Expenses</span><span class="expenses-output">$<?php echo $expense; ?></span></div>
        <div><span class="net-profit-label">Net Profit</span><span class="net-profit-output">$<?php echo $netprof; ?></span></div>
        <br><br><input id="submit" class="button button-primary" type="submit" value="Calculate"></input>
    </form>
<?php }