点击提交（表单提交）后，系统会抛出错误

Question

点击提交时，系统将引发错误，

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HTML（index.html）。

我已经为form.php创建了一个单独的文件，用于PHP表单提交（提供给action属性）

<form method="post" action="form.php" class="contact-form row" data-sal="zoom-in" data-sal-duration="800">

      <div class="form-field col x-50" id="contact_page">
        <input type="text" id="name" class="input-text" name="sender_name" required>
        <label for="name" class="label">Name</label>
      </div>

      <div class="form-field col x-50">
        <input type="email" id="email" class="input-text" name="sender_mail" required>
        <label for="email" class="label">Email</label>
      </div>

      <div class="form-field col x-100">
        <input type="text" id="message" class="input-text" name="sender_message" required>
        <label for="message" class="label">Message</label>
      </div>

      <div class="form-field col x-100 align-center">
        <input type="submit" name="submit" class="submit-btn" value="Submit">
      </div>

    </form>

我认为不需要CSS

PHP

<?php
if($_POST["submit"]) {
    $recipient="subinsamrat96@gmail.com";
    $subject="Form to email message";
    $sender=$_POST["sender_name"];
    $senderEmail=$_POST["sender_mail"];
    $message=$_POST["sender_message"];

    $mailBody = "Name: $sender\nEmail: $senderEmail\n\n $message";

    mail($recipient, $subject, $mailBody, "From: $sender <$senderEmail>");

    $thankYou = "<p>Thank you! Your message has been sent. </p>";
}

?>