Slide Captcha

A captcha is a test that can be used to determine whether the user is a human to prevent bot and spam attacks. This specific captcha uses a puzzle that has to be solved by pulling the slider into the correct position. In contrast to other graphical captchas, this captcha also verifies the result on the server side. Another advantage is that no jQuery is required. Only PHP and JavaScript.

Installation
Copy SlideCaptcha.php, SlideCaptcha.js, SlideCaptcha.css and the folder SlideCaptcha and it's content in the root directory of the web. Link SlideCaptcha.js and SlideCaptcha.css on the website where you want to use the captcha, as follows:

<script type="text/javascript" src="SlideCaptcha.js"></script>
<link rel="stylesheet" type="text/css" href="SlideCaptcha.css"/>

Follow one of the following examples.

Example 1
This example shows, how to use the captcha with a form-submit and the captcha on the submit-button. Insert an onclick-listener in the button-tag:

<button onclick="javascript:captcha('testform');return false;">Submit</button>

Add the JavaScript:

<script>
  function captcha(form) {
    slideCaptcha.onsuccess(function () {
      document.getElementById(form).submit();
      slideCaptcha.hide();
      slideCaptcha.refresh();
    });
    slideCaptcha.init();
    slideCaptcha.show();
  }
</script>

The form is not submitted by clicking on the button. Instead, the captcha is displayed. If the captcha has been solved correctly, the form is submitted. Check again on the server side, whether the captcha has really been solved:

<?php
  session_start();
  if ((isset($_POST['send'])) && ($_POST['send'] == '1')) {
    if ((isset($_SESSION['SlideCaptchaOk'])) && ($_SESSION['SlideCaptchaOk'] == 'ok')) {
      unset($_SESSION['SlideCaptchaOk']);
      echo '<p style="color:lime">Form submit received: Captcha solved</p>';
    } else {
      echo '<p style="color:red">Form submit received: Captcha NOT solved.</p>';
    }
  }
?>

Example 2
This example shows, how to use the captcha with a form-submit and the captcha on a checkbox. Insert an onclick-listener in the checkbox-tag:

<input id="id_checkbox" type="checkbox" required="required" onclick="javascript:captcha('id_checkbox');return false;" />

Add the JavaScript:

<script>
  function captcha(checkbox) {
    slideCaptcha.init();
    slideCaptcha.show();
    slideCaptcha.onsuccess(function () {
      document.getElementById(checkbox).checked = true;
      slideCaptcha.hide();
      slideCaptcha.refresh();
    });
  }
</script>

By using the required option inside the checkbox-tag, the form can only be submitted when the checkbox is checked. By clicking on the checkbox, the captcha is displayed. If the captcha has been solved correctly, the checkbox will be checked and the form can be submitted. Check again on the server side, as described in example 1, whether the captcha has really been solved.

Example 3
This example shows, how to use the captcha with a hyperlink. Insert an onclick-listener in the hyperlink-tag. Also move the href link into the JavaScript-function:

<a href="" onclick="javascript:captcha('Example3.php');return false;">DOWNLOAD</a>

Add the JavaScript:

<script>
  function captcha(file) {
    slideCaptcha.onsuccess(function () {
      window.location.href = file;
      slideCaptcha.hide();
      slideCaptcha.refresh();
    });
    slideCaptcha.init();
    slideCaptcha.show();
  }
</script>

The captcha is displayed by clicking on the hyperlink. If the captcha has been solved correctly, the JavaScript will redirect to the specified location. Check again on the server side, whether the captcha has really been solved and deliver the content:

<?php
  session_start();
  if ((isset($_SESSION['SlideCaptchaOk'])) && ($_SESSION['SlideCaptchaOk'] == 'ok')) {
    unset($_SESSION['SlideCaptchaOk']);
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename="Example3.php"');
    readfile('Example3.php');
    die();
  }
?>

Error tolerance
In order to offer better user experience, an error tolerance can be specified in SlideCaptcha.php.

errorTolerance of 0 means zero tolerance
errorTolerance of 3 means +- 3 pixel tolerance (recommended)

Photos
All photos provided by unsplash.com. As described on their website, all photos, offered on their website, can be used for free for commercial and non-commercial purposes:

Download:

Change log:

Version 1: Initial version

Published: 8. Feb 2024
Categories: PHP HTML Javascript