HTTP/1.1 401 使用 XML-RPC PHP 客户端和服务器时未授权

Question

HTTP/1.1 401 Unauthorized

This server could not verify that you are authorized to access the document
you requested.  Either you supplied the wrong credentials (e.g., bad 
password), or your browser does not understand how to supply the 
credentials required. 

Fault Code: 5, Reason: Didn't receive 200 OK from remote server. 
(HTTP/1.1 401 Unauthorized) 

我正在尝试通过 PHP XML-RPC 协议通过其 IP 地址访问 API 服务器。为什么服务器会给出此错误？