Manual:$wgEnableMWSuggest

Details
Enable AJAX autocomplete search suggestions (autosuggest) 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.

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

Usage
In LocalSettings.php, one may set: $wgEnableMWSuggest = true;