Stelian/supervisord-php-client — pull request #1 by ofetisov (2014-07-07 13:17:51+00:00)
Title: HTTP protocol version change

There is a problem with your extension. HTTP v1.1 requires "Host" header to be send. When working directly with supervisors server - everything is alright, but when supervisor is behind nginx there is an issue. Nginx returns code 400 error with the following message in the log:
*1 client sent HTTP/1.1 request without "Host" header while reading client request headers, client: xxx , server: , request: "POST /RPC2 HTTP/1.1

So there is two solutions, and the easiest is to change HTTP protocol version.