API申请地址：https://api.mch.weixin.qq.com/v3/ecommerce/applyments/

/**
 *获取证书
 **/
public function getzhengshu(){
    $url="https://api.mch.weixin.qq.com/v3/certificates";//获取地址
    $timestamp = time();//时间戳
    $nonce = $this->nonce_str();//获取一个随机字符串
    $body = "";
    $mch_private_key = $this->getPrivateKey($this->keyPath); //调用获取商户私钥方法传证书文件路径进去
    $merchant_id = $this->mc_id; //服务商商户号
    $serial_no = $this->serial_no; //服务商证书序列号
    $sign=$this->sign($url,'GET',$timestamp,$nonce,$body,$mch_private_key,$merchant_id,$serial_no);

    $header=[
        'Authorization:WECHATPAY2-SHA256-RSA2048 '.$sign,
        'Accept:application/json',
        'User-Agent:'.$merchant_id
    ];
    $result = $this->curl($url,'',$header,'GET');
    $result = json_decode($result,true);

    $serial_no = $result['data'][0]['serial_no'];//获取的平台证书序列号
    $encrypt_certificate = $result['data'][0]['encrypt_certificate'];
    $sign_key = $this->APIKeyV3;  //APIv3密钥，商户平台API安全中获取
    $result = $this->decryptToString($encrypt_certificate['associated_data'],$encrypt_certificate['nonce'],$encrypt_certificate['ciphertext'],$sign_key);

    if(!is_file(EASYSWOOLE_ROOT.'/cert/partner/temp_cert.pem')) {
        file_put_contents(EASYSWOOLE_ROOT.'/cert/partner/temp_cert.pem',$result);
    }
    return $serial_no;//返回平台证书序列号
}

//申请进件
public function add($param){
    $url = 'https://api.mch.weixin.qq.com/v3/ecommerce/applyments/';
    $merchant_id = $this->mc_id;//服务商商户号
    $serial_no = $this->serial_no;//服务商证书序列号
    $mch_private_key = $this->getPrivateKey($this->keyPath);//服务商API证书路径

    $timestamp = time();
    $nonce = $this->nonce_str();
    $sign = $this->sign($url,'POST',$timestamp,$nonce,json_encode($param),$mch_private_key,$merchant_id,$serial_no);//进行签名操作

    //设置header
    $header = [
        //设置发送的头信息
        'Authorization:WECHATPAY2-SHA256-RSA2048 ' . $sign,//签名
        'Accept:application/json',
        'User-Agent:' . $merchant_id,//服务商的商户号
        'Content-Type:application/json',
        'Wechatpay-Serial:' . $this->getzhengshu() //获取平台证书的序列号
    ];

    $result = $this->curl($url,json_encode($param),$header);
    $result = json_decode($result, true);
    return $result;
}

/**
 *解密方法
 */
public function decryptToString($associatedData,$nonceStr,$ciphertext,$aesKey){
    $ciphertext= base64_decode($ciphertext);
    if(function_exists('sodium_crypto_aead_aes256gcm_is_available')&& sodium_crypto_aead_aes256gcm_is_available()){
        return sodium_crypto_aead_aes256gcm_decrypt($ciphertext,$associatedData,$nonceStr,$aesKey);
    }
    if(PHP_VERSION_ID >= 70100 && in_array('aes-256-gcm', openssl_get_cipher_methods())){
        $ctext=substr($ciphertext,0,-16);
        $authTag=substr($ciphertext,-16);
        return openssl_decrypt(
            $ctext,
            'aes-256-gcm',
            $aesKey,
            \OPENSSL_RAW_DATA,
            $nonceStr,
            $authTag,
            $associatedData
        );
    }
    throw new \RuntimeException('php7.1');
}

/**
 *加密字符串
 * @param $str 要加密的字符串
 * @return string
 * @throws \Exception
 */
public function getEncryptS($str) {
    $public_key_path = EASYSWOOLE_ROOT.'/cert/partner/temp_cert.pem';
    $public_key = file_get_contents($public_key_path);
    $encrypted  = '';
    if (openssl_public_encrypt($str, $encrypted, $public_key, OPENSSL_PKCS1_OAEP_PADDING)){
        $sign = base64_encode($encrypted);
    }else{
        throw new \Exception('encrypt failed');
    }

    return $sign;
}

//最终返回结果
Array
(
    [code] => PARAM_ERROR
    [message] => 证书类型格式错误，请重新选择
)

//所以是哪个证书问题？已确认服务商API证书没有问题