Manual:$wgHTTPProxy

Details
Proxy to use for cURL (or PHP fopen_url if cURL not available) requests, for example in config>Special:MyLanguage/Manual:Configuring file uploads#Uploading_directly_from_a_URL_.28.22Sideloading.22.29|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.