private function writeSession($message, $from = '', $is_error = false){
        if( session_id() ){
            $key = $is_error ? $this->error_key : $this->message_key;
            // Initialize
            if( !isset($_SESSION[$key]) || !is_array($_SESSION[$key]) ){
                $_SESSION[$key] = [];
            }
            // Add message
            $_SESSION[$key][] = ( empty($from) ? '' : sprintf('<strong>[%s]</strong> ', $from) ).$message;
        }
    }