I try to use OAuth Provider in PHP (PECL Package) behind a apache reverse-proxy
the client uses
POST https://api.com/resource/oauth/request-token
I had to do this once. I ended up modifying the OAuth code to pass along the actual URL the provider will receive as well the URL I need to send to from behind my proxy. The former was used in the signature and the latter in the HTTP request. It was a pain and not portable (if anything changed in the proxy, the code would stop working)