微信支付沙箱测试一直提示 ”请确认请求参数是否正确param mch_id invalid"“

https://api.mch.weixin.qq.com/sandboxnew/pay/getsignkey

post请求时添加上header头Content-Type: application/xml

curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/xml'));