注意：未定义的索引：变量

我正在尝试构建以下形式：

<form method="post" action="Index.php">
  <label>Name </label>
  <input name="name" placeholder="Type Here"><br />
  </br>
  <label>Email </label>
  <input name="email" placeholder="Type Here">
  <br /></br>
  <label style="display:block;">I need some information regarding:</label>
  <textarea name="message" placeholder="Type Here"></textarea>
  <br />
  <label>*What is 2+2? (Anti-spam)</label>
  <input name="human" placeholder="Type Here">
  <br />
  <input id="submit" name="submit" type="submit" value="Submit" style="height:30px;">
</form>

PHP代码：

<?php
  $name = $_POST['name'];
  $email = $_POST['email'];
  $message = $_POST['message'];
  $from = 'From: TangledDemo';
  $to = '[email protected]';
  $subject = 'Hello';
  $human = $_POST['human'];

  $body = "From: $name\n E-Mail: $email\n Message:\n $message";

  if ($_POST['submit'] && $human == '4') {
    if (mail ($to, $subject, $body, $from)) {
      echo '<p>Your message has been sent!</p>';
    } else {
      echo '<p>Something went wrong, go back and try again!</p>';
    }
  } else if ($_POST['submit'] && $human != '4') {
    echo '<p>You answered the anti-spam question incorrectly!</p>';
  }

  if ($_POST['submit']) {
    if ($name != '' && $email != '') {
      if ($human == '4') {
        if (mail ($to, $subject, $body, $from)) {
          echo '<p>Your message has been sent!</p>';
        } else {
          echo '<p>Something went wrong, go back and try again!</p>';
        }
      } else if ($_POST['submit'] && $human != '4') {
        echo '<p>You answered the anti-spam question incorrectly!</p>';
      }
    } else {
      echo '<p>You need to fill in all required fields!!</p>';
    }
  }
?>

但是不断出现这些错误：

注意：未定义的索引：第19行的C：\ xampp \ htdocs \ bet4info \ Index.php中的名称

注意：未定义的索引：第20行的C：\ xampp \ htdocs \ bet4info \ Index.php中的电子邮件

注意：第21行的C：\ xampp \ htdocs \ bet4info \ Index.php中的消息未定义索引：

注意：未定义的索引：第25行的C：\ xampp \ htdocs \ bet4info \ Index.php中的人员

注意：未定义的索引：在第29行的C：\ xampp \ htdocs \ bet4info \ Index.php中提交

注意：未定义的索引：在第35行的C：\ xampp \ htdocs \ bet4info \ Index.php中提交

注意：未定义的索引：在第38行的C：\ xampp \ htdocs \ bet4info \ Index.php中提交

为什么会这样呢？