chamilo-lms2/main/inc/ajax/form.ajax.php

<?php
/* For licensing terms, see /license.txt */
//require_once '../global.inc.php';

$action = isset($_REQUEST['a']) ? $_REQUEST['a'] : null;

switch ($action) {
    case 'get_captcha':
        header('Content-Type: image/jpeg');

        $sessionVar = (empty($_REQUEST['var']))
                      ? '_HTML_QuickForm_CAPTCHA'
                      : $_REQUEST['var'];
        // Force a new CAPTCHA for each one displayed

        /** @var Text_CAPTCHA $obj */
        $obj = $_SESSION[$sessionVar];
        $obj->generate(true);
        echo $image = $obj->getCAPTCHA();

        //echo $_SESSION[$sessionVar]->getCAPTCHAAsJPEG();
        exit;
        break;
}