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).

If this setting is not set, MediaWiki may fallback to using the  environment variable if set.