Omnipay\Alipay\Common\Signer::prefix PHP Method

    private function prefix($key)
    {
        if (strtoupper(substr(PHP_OS, 0, 3)) != 'WIN' && is_file($key) && substr($key, 0, 7) != 'file://') {
            $key = 'file://' . $key;
        }
        return $key;
    }