howto include in another page:
  <table><tr><td><img src="./captcha.php"></td>
             <td> <?php echo _("Please fill in the code displayed here"); ?><br>
                  <input type="text" name="captcha_check"></td>
  </table>

and to check the result in the form submit page:

  require_once(ESP_BASE.'public/captcha_check.php');
  $msg = response_check_captcha("captcha_check",1);
  if(!empty($msg)) {
    ... print your error
    exit;
  }

The function response_check_captcha() receives 2 parameters:
- first: the name of the input field that has the captcha value
- second:
      whether or not to cleanup the captcha session var and the input var
      after positive evaluation (so the captcha is correct). This prevents
      from entering the same value again by eg. reloading the page.
      Possible values: 0|1 (default: 1, cleanup)
      In some cases the cleanup is not immediately desired (like the way I
      use it in phpESP, so that's why this variable is there).