private function _generateAlphaToken()
    {
        $timestamp = strtotime($this->_generateTimestamp());
        $characters = str_split('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ');
        shuffle($characters);
        $randomString = '';
        foreach (str_split((string) $timestamp) as $num) {
            $randomString .= $characters[$num];
        }
        return $randomString;
    }