// If a search results returns only a single post, then redirect the user to that post
add_action( 'template_redirect', function () {
if ( is_search() ) {
global $wp_query;
if ( $wp_query->post_count == 1 && $wp_query->max_num_pages == 1 ) {
wp_redirect( get_permalink( $wp_query->posts['0']->ID ) );
exit;
}
}
} );