Manual:$wgEnableMWSuggest

Details
Enable suggestions while typing in search boxes (results are passed around in OpenSearch format). You might have to comment out $wgScriptExtension = ".php5"; in your LocalSettings.php to make this work.

In LocalSettings.php, also include $wgEnableMWSuggest = true;

This setting replaces $wgAjaxSearch that was used in earlier versions of MW.