我用提交按钮制作了联系表格。我希望保存所有已输入联系表的信息。提交按钮以某种方式不起作用。

过去它似乎可以工作，但是作为php / html新手，我似乎找不到问题

HTML

        <form method="post">
            <div class="contact-form" action="contact-form.php" method="post">
                <div class="contentLeft">
                    <h4>Ihre Daten</h4>
                    <br>
                    <h3>Vorname</h3>
                    <input class="inputText" type="text" name="firstName" required="required">
                    <br>
                    <h3>Nachname</h3>
                    <input class="inputText" type="text" name="lastName" required="required">
                    <br>
                    <h3>E-Mail</h3>
                    <input class="inputText" type="text" name="mail" required="required">
                    <br>
                    <h3>Straße</h3>
                    <input class="inputText" type="text" name="street" required="required">
                    <br>
                    <h3>Postleitzahl</h3>
                    <input class="inputText" type="text" name="postal" required="required">
                    <br>
                    <h3>Stadt</h3>
                    <input class="inputText" type="text" name="city" required="required">
                    <br>
                    <h3>Telefonnummer</h3>
                    <input class="inputText" type="number" name="phone" required="required">
                    <br>
                </div>
                <div class="contentRight">
                    <h4>Ihre Nachricht</h4>
                    <br>
                    <h3>Betreff</h3>
                    <input class="inputTextRight" type="text" name="subject" required="required">
                    <br>
                    <h3>Buchungscode</h3>
                    <input class="inputTextRight" type="text" name="bookingCode" required="required">
                    <br>
                    <br><
                    <textarea></textarea>
                </div>
                <br>
                <button class="button" type="submit" name="submit">Absenden</button>
            </div>
        </form>

PHP

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

    $firstName = $_POST['firstname'];
    $lastName = $_POST['lastname'];
    $mail = $_POST['mail'];
    $street = $_POST['street'];
    $postal = $_POST['postal'];
    $city = $_POST['city'];
    $phone = $_POST['phone'];
    $subject = $_POST["subject"];
    $bookingCode = $_POST['bookingCode'];
    $message = $_POST['message'];
    $data=$_POST["firstName"] ."\n".$_POST["firstName"] ."\n".$_POST["lastName"] ."\n".$_POST["mail"] ."\n".$_POST["street"] ."\n". $_POST["postal"] ."\n".$_POST["city"] ."\n".$_POST["phone"] ."\n". $_POST["subject"] ."\n".$_POST["bookingCode"] ."\n". $_POST["message"];
    $fp = fopen("data.txt","a");
    fwrite($fp,$data);
    fclose($fp);
    header ("Location: NewTest.html?mailsent");
}
?>