I’ve packaged the functionality inside a WordPress plugin called is_child that you can download here. Unzip it, upload it to wp-content/plugins and activate the plugin.

Coding is_child

Doing a quick web search brings up tons of sites providing some sort of is_child or is_tree function. Most of the time it’s something like the following:

function is_child($parent) {
  global $post;
  return $post->post_parent == $parent;
}

This is great and a pretty nice solution. I’ve taken this approach and extended it with the following functionality:

• by default we’ll retrieve all parent pages up to the root node and test whether the current page is part of this tree. This behavior can be disabled though.
• you can supply the name (slug) of a post/page instead of the ID and the function will look it up for you.

All this combined with some error checking makes a great idea even better. Just have a look at the code if you would like to know more about the exact implementation.

Conclusion

We’ve seen that it’s very easy to test whether the current page is a child of another page or part of a certain tree in your page hierarchy. Packaging this into a plugin or placing the necessary code into your theme’s functions.php allows you to display certain content on particular pages only. In case you’re using the Widget logic plugin you now can display widgets on pages belonging to a particular subtree of your pages only, which is great.