It's X-Forwarded-For. Although I'm not sure if PHP would convert hyphens to underscores. Web-based proxies won't pass a X-Forwarded-For, but normal proxies, such as those on 8080, should do.

The XFF project may of be some use to you in this case.

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The man who comes back through the Door in the Wall will never be quite the same as the man who went out.