public function build($length = null, $charset = null)
    {
        if ($length !== null) {
            $this->length = $length;
        }
        if ($charset !== null) {
            $this->charset = $charset;
        }

        $phrase = '';
        $chars = str_split($this->charset);

        for ($i = 0; $i < $this->length; $i++) {
            $phrase .= $chars[array_rand($chars)];
        }

        return $phrase;
    }