We can add captcha for blocking spams robots accessing our site or application. We can add captcha in our web application and website by using the below code ->

<?php
App::uses('AppController', 'Controller');
class PagesController extends AppController {
    public $name = 'Pages';
    public $uses = array();
    public function display() {

    if (!empty($this->request->data)) { // this isused to save the form

      session_start(); //        start session so we can access the data saved by captcha.php */

      if ($this->request->data['Pages']['captcha'] == $this->Session->read('captcha')) {
      $this->set('pass', true);  /* set $pass for view to true */

      } else { 

        $this->set('pass', false);
      }
    }


    $this->render('home');// display home file under view folder contains forms and captcha
    }
}

make a new home.ctp in view folder and paste the below code

<?php
/* access control not performed yet */
if (!isset($pass)) {
echo $this->Form->create('Pages');
?>
<p>Enter the words displayed in image</p>
<!-- when browser gets captcha.php which generates image, proper value for captcha will be saved in session -->
<img src="/app/webroot/captcha/captcha.php" id="captcha" /><br/>
<?php
echo $this->Form->input('captcha');
echo $this->Form->submit('Let me in');
} else { 
  if ($pass) {
  ?>
  <p style="color: green">OK. You're loggedin.</p>
  <button onclick="document.location='/'">Go back</button>
  <?php
  } else {
  ?>
  <p style="color: red">Go away spammer!</p>
  <?php
  }
}
?>