提示错误：Cannot load privateKey from(string), please take care about the \$thing input.

/**
 * 构建一个客户端实例
 * wechatpay-php
 */
public static function instanceWeChatPay()
{
    // 商户号
    $merchantId = WeChat::$merchantId;
    // 从本地文件中加载商户API私钥，商户API私钥会用来生成请求的签名
    $merchantPrivateKeyInstance = Rsa::from(WeChat::$merchantPrivateKeyFilePath, Rsa::KEY_TYPE_PRIVATE);
dd(  $merchantPrivateKeyInstance  );
    // 商户API证书序列号
    $merchantCertificateSerial = WeChat::$merchantCertificateSerial;

    // 从本地文件中加载微信支付平台证书，用来验证微信支付应答的签名
    $platformPublicKeyInstance = Rsa::from(WeChat::$platformCertificateFilePath, Rsa::KEY_TYPE_PUBLIC);
    // 获取微信支付平台证书序列号
    $platformCertificateSerial = PemUtil::parseCertificateSerialNo(WeChat::$platformCertificateFilePath);

    // 构造一个 APIv3 客户端实例
    WeChat::$instance = Builder::factory([
        'mchid'      => $merchantId,
        'serial'     => $merchantCertificateSerial,
        'privateKey' => $merchantPrivateKeyInstance,
        'certs'      => [
            $platformCertificateSerial => $platformPublicKeyInstance,
        ],
    ]);
}