Manual:$wgSearchForwardUrl

Details
Set this to a URL to forward search requests to some external location. If the URL includes ' ', this will be replaced with the URL-encoded search term. For example, to forward to Google you'd have something like:

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

NOTE: 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.