Post Ancestor and Child Post in Custom Post Type

^{Hierarchical structure of custom post type “Book” (for example).}

When we are on Post 2-95, I want to know:

• Does the post has this post ancestor(Post 1-31)?
• Does it have child posts(Post 3-19, Post 3-10)?

Then, if it has:

• an ancestor post: retrieve (object) of this post.
• a child posts: retrieve (objects) of these posts.

Given a post represented by a post object $p, you can find out if post 31 is the parent via:

if($p->post_parent == 31){
    // it is!
} else {
    // it isn't
}

To figure out the children, something like:

$posts = get_posts(array(
    'post_parent' => $p->ID,
    'post_type'   => $p->post_type
));
// if there are children, they will be contained in `$posts`

Finally, to determine how many levels deep down the hierarchy you are, you will need to recurse up the hierarchy $p->parent_post == 0, and then count how many times you needed to do that.

e.g.

$level = 1;
while($parent->parent_post != 0){
    $level++;
    $parent = get_post($parent->parent_post);
}