PHP form protection tips: How to prevent repeated form submissions

When using PHP forms for data submission, the problem of repeated form submission often occurs. This can lead to inaccurate data or, worse, system crashes. Therefore, it is very important to understand how to prevent duplicate submissions. In this article, I will introduce some PHP form protection techniques to help you effectively prevent repeated form submission problems.

1. Add a token to the form

Adding a token to the form is a common way to prevent repeated submissions. This method works by adding a hidden field to the form that contains a unique random value. When the form is submitted, the PHP code will check if this value matches the value stored on the server. If there is no match, it means that the form has been submitted repeatedly and the data will not be processed. The following is a basic example of token usage:

<?php
  session_start();
  if ($_SERVER['REQUEST_METHOD'] === 'POST' && !empty($_POST['token'])) {
    $token = $_POST['token'];
    if ($_SESSION['token'] === $token) {
      //处理表单数据
      //...
      unset($_SESSION['token']);
    } else {
      echo '表单已提交！';
    }
  } else {
    $token = md5(uniqid(rand(), true));
    $_SESSION['token'] = $token;
?>
  <form method="post">
    <input type="hidden" name="token" value="<?php echo $token; ?>" />
    <!--其他表单元素-->
    <button type="submit">提交</button>
  </form>
<?php } ?>

In this example, we first check whether the request is POST, and if so, check whether the token value in the form matches. If the tokens match, process the form data. After processing the data, we use the unset function to remove the token from the SESSION because this token is no longer needed. If the tokens do not match, the form has been submitted twice and the form data will not be processed. If the request is not a POST, we generate a new token, store it in the SESSION, and add it to the form.

2. Use redirection

After submitting the form, if you still use the same URL, it will cause the form to be submitted repeatedly. To avoid this, you can use redirects. This technique involves an intermediary page that redirects the user to another page after the form is submitted, rather than the original form page. The following is an example of using redirection to prevent repeated form submission:

<?php
  if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    //处理表单数据
    //...
    header('Location: success.php');
    exit;
  }
?>

In this example, we use the header function to redirect the user to the success.php page after processing the form data. This page is a page that is displayed after a successful submission. It does not contain any form, so the user cannot resubmit the form using the browser's "back" button or reloading the page.

3. Disable browser caching

Caching the form in the browser may cause the form to be submitted repeatedly. Because browsers can cache a site's Web pages, the form may be resubmitted when the user clicks the browser's Back button. To avoid this, you need to disable browser caching. The following is a basic example of disabling browser caching:

<?php
  header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");         // Date in the past
  header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT"); // always modified
  header("Cache-Control: no-store, no-cache, must-revalidate");  // HTTP/1.1
  header("Cache-Control: post-check=0, pre-check=0", false);
  header("Pragma: no-cache");                                // HTTP/1.0
?>

In this example, we use several HTTP headers to inform the browser not to cache the current page. This includes an "Expires" header for dates in the past, and several other headers about how to process the page.

Conclusion

When using PHP forms, it is very important to prevent repeated submissions. The above methods describe some effective techniques that can help you avoid repeated submission of forms and protect your data security. If you want to learn more about PHP form defense, check out the PHP documentation or refer to blogs and articles from other PHP developers.

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