slug link change

slug

function philosophy_cpt_slug_fix($post_link, $id){
    $p = get_post($id);
    if(is_object($p) && 'chapter'==get_post_type($id)){
        $parent_post_id = get_field('parent_book');
        $parent_post = get_post($parent_post_id);
        if($parent_post){
            $post_link = str_replace("%book%", $parent_post->post_name,$post_link);
        }
    }
            return $post_link;
}
add_filter('post_type_link', 'philosophy_cpt_slug_fix', 1,2);