bootstrap 4 表单返回值问题

Question

我正在使用 bootstrap 4 构建一个站点，并添加了一个表单。我把它放在我的服务器上并运行它，它可以工作！有点。有4个字段，名字，姓氏，电子邮件，电话。验证有效，我已将其发送到电子邮件并且有效，但无论在网站的输入字段中输入什么，电子邮件都会返回：

Name: S S.
Email: S.
Phone: S.

这是我的 php：

<?php
$first_name= S_POST['first_name'];
$last_name= S_POST['last_name'];
$email= S_POST['email'];
$phone= S_POST['phone'];

$email_from =' Company, llc';
$email_subject = 'New Message From A Guest';
$email_body = "Name: $first_name $last_name.\n".
              "Email: $email.\n".
              "Phone: $phone.\n";
$to ="me@business.com";
$headers = "From: $email_from \r\n";
$headers .= "Reply-To: $email \r\n";

mail($to,$email_subject,$email_body,$headers);
header("location: index.html");
?>

表单 HTML：

 <form class="form inline d-flex justify-content-center" action="contact.php" method="POST" role="form">
             <br style="clear:both">
            <div class="col-sm-6">
              <div class="form-group">
                <input type="text" class="control" id="first_name" name="first_name" placeholder="First Name" required>
              </div>
              <div class="form-group">
                <input type="text" class="control" id="email" name="email" placeholder="Email" required>
              </div>
            </div>
            <div class="col-sm-6">
              <div class="form-group">
                <input type="text" class="control" id="last_name" name="last_name" placeholder="Last Name" required>
              </div>
              <div class="form-group">
                <input type="text" class="control" id="phone" name="phone" placeholder="Phone" required>
              </div>
            </div>
            <div class="col-sm-12">
              <button type="submit" id="submit" name="submit" class="btn btn-primary">SEND</button>
          </form>

什么可能导致它只返回 S 作为这些输入的值？任何帮助将不胜感激

Answer 1

$first_name= S_POST['first_name'];
$last_name= S_POST['last_name'];
$email= S_POST['email'];
$phone= S_POST['phone'];

不应该是“S_”，而应该是美元 $_

如：

$first_name= $_POST['first_name'];
$last_name= $_POST['last_name'];
$email= $_POST['email'];
$phone= $_POST['phone'];