表单操作未触发，而是重定向到 php 脚本

对于一件小事来说，您的HTML很好，在点击提交之后，您没有指定将表单发送到哪里.这是通过在<form>中的action参数内指定PHP脚本来实现的

<form method="post" action="form-to-email.php">
    <div class="input-group">
        <input type="text" id="name" name="name" class="input-demo" placeholder="Your Name">
        <span id="invalid-name">
            Please enter at least 2 chars
        </span>
    </div>
    <div class="input-group">
        <input id="email" type="email" name="email" class="input-demo" placeholder="Email Address">
        <span id="invalid-email">
            Please enter valid email
        </span>
    </div>
    <div class="input-group">
        <textarea id="message" name="message" placeholder="Message">
        </textarea>
        <span id="invalid-message">
            Please write something for us
        </span>
    </div>
    <div>
    </div>
    <input type="submit" name="submit" value="Book a Demo">
</form>

在阅读了别人的回答上的问题后，我终于找到了你想要的，你可以通过在脚本的末尾添加一个标题(Location:index.html)来阻止网站停留在你的phpscript页面上，这样当它完成注册或失败时，你可以将它发送回索引.或者，您可以在索引文件中包含PHP，但您必须将索引从.html更改为.php

在php脚本中使用重定向的示例

<?php
if (isset($_POST['submit'])) {
  include 'index.php';
  $to = "email@example.com"; // this is your Email address
  $from = $_POST['email']; // this is the sender's Email address
  $name = $_POST['name'];
  $subject = "Form submission";
  $subject2 = "Copy of your form submission";
  $message = $name . " wrote the following:" . "\n\n" . $_POST['message'];
  $message2 = "Here is a copy of your message " . $first_name . "\n\n" . $_POST['message'];

  $headers = "From:" . $from;
  $headers2 = "From:" . $to;
  mail($to, $subject, $message, $headers);
  mail($from, $subject2, $message2, $headers2); // sends a copy of the message to the sender
  echo "Mail Sent. Thank you " . $name . ", we will contact you shortly.";
  header('Location: index.html');
} else {
  echo 'isset was false';
  header('Location: index.html');
}
?>