Manual:$wgHTTPProxy/ru

Подробнее
Proxy to use for cURL (or PHP fopen_url if cURL not available) requests, for example in side-uploads (uploads from an external URL). Format is: IP address of the proxy, a colon, and the port number.

Since August 2016, MediaWiki will not fallback to using the  environment variable, due to the vulnerability called httpoxy.