Redirect WordPress' native search to an external engine, like Google's Programmable Search

functions.php

function redirect_search() {
    if (is_search() && !empty($_GET['s'])) {
		  wp_redirect('https://cse.google.com/cse?cx=PSE_ID&q=' . urlencode(get_query_var('s')) );
      exit();
    }
}
add_action('template_redirect', 'redirect_search' );