Add a topic-based search feature to your bbPress forums with the following PHP code snippet. This will enable users to search specifically for topics within your forums:

<?php
function bbpress_search_by_topic( $query ) {
    if ( !is_admin() && is_search() && isset( $_GET['forum_id'] ) ) {
        $query->set( 'post_type', 'topic' );
    }
}
add_action( 'pre_get_posts', 'bbpress_search_by_topic' );
?>

This code ensures that search results are focused specifically on topics within bbPress forums, providing more relevant search results for users.