protected function buildBitCoinString()
    {
        $query = http_build_query([
            'amount'    => $this->amount,
            'label'     => $this->label,
            '$message'  => $this->message,
            'r'         => $this->returnAddress,
        ]);

        $btc = $this->prefix.$this->address.'?'.$query;

        return $btc;
    }