PHP - Sum Two Numbers

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
<?php if (isset($_GET['firstNumber']) &&
    isset($_GET['secondNumber'])) {
    $num1 = intval($_GET['firstNumber']);
    $num2 = intval($_GET['secondNumber']);
    $sum = $num1 + $num2;

    echo "$num1 + $num2 = $sum";
}
?>
    <form>
        <div>First Number:</div>
        <input type="number" name="firstNumber"/>
        <div>Second Number:</div>
        <input type="number" name="secondNumber"/>
        <div><input type="submit" value="Submit"/></div>
    </form>
</body>
</html>