Contao\Form::initializeSession PHP Method

    protected function initializeSession($formId)
    {
        if (\Input::post('FORM_SUBMIT') != $formId) {
            return;
        }
        $arrMessageBox = array('TL_ERROR', 'TL_CONFIRM', 'TL_INFO');
        $_SESSION['FORM_DATA'] = is_array($_SESSION['FORM_DATA']) ? $_SESSION['FORM_DATA'] : array();
        foreach ($arrMessageBox as $tl) {
            if (is_array($_SESSION[$formId][$tl])) {
                $_SESSION[$formId][$tl] = array_unique($_SESSION[$formId][$tl]);
                foreach ($_SESSION[$formId][$tl] as $message) {
                    /** @var FrontendTemplate|object $objTemplate */
                    $objTemplate = new \FrontendTemplate('form_message');
                    $objTemplate->message = $message;
                    $objTemplate->class = strtolower($tl);
                    $this->Template->fields .= $objTemplate->parse() . "\n";
                }
                $_SESSION[$formId][$tl] = array();
            }
        }
    }