Manual:$wgSearchForwardUrl

Search: $wgSearchForwardUrl
Set this to a URL to forward search requests to an external location.
Introduced in version:1.4.0
Removed in version:Still in use
Allowed values:(URL string)
Default value:null

Details

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Set this to a URL to forward Special:Search to an external location. If the URL includes $1, this will be replaced with the URL-encoded search term. For example, to forward to DuckDuckGo you could set the following (below $wgServer) in LocalSettings.php:

$wgSearchForwardUrl = 'https://duckduckgo.com/?q=$1&sites=' . $wgServer;

You also need to set $wgDisableTextSearch = true; or $wgSearchForwardUrl is ignored.

From versions 1.4.8 to 1.4.15, this setting was incorrectly named $wgForwardSearchUrl in DefaultSettings.php. If you are using one of those versions then this alternative name should be used instead.