WordPress Quick Tip - Is Page Downstream

Using WordPress conditional tags can be quite helpful when working within plugins or themes. For instance, within the loop, you might want to know if you are dealing with a specific page:

if ( is_page(13) ) {
    //do some stuff to page with id of 13
}

Pretty simple and everyone knows this. But what if I want to know if I am dealing with a child of page 13? Also pretty easy:

if ( $post->post_parent == 13 ) {
    //do some stuff to a direct child of page 13
}

Combine both to check if we are dealing with either page 13 or a child of page 13:

if ( is_page(13) || $post->post_parent == 13 ) {
    //do some stuff to page 13 or children of page 13
}

But I want to go one step further! What if I want to know if I am dealing with a child of a child of page 13? And I don’t care how many levels deep, so it could be a child of a child of a child…

This is what you could use:

if ( in_array(13, $post->ancestors) ) {
    //do some stuff to a child of page 13 (any level deep)
}

(Obviously, this will also return true for direct children of page 13)

Now, let’s combine all this together into a useful helper function:

function is_page_downstream($id) {
    global $post;
    if (!is_page()) return false;
    return is_page($id) || in_array($id, $post->ancestors);
}

An example

Working with the below page structure, I want to know when I am on page ‘Top Level’ or any downstream page underneath ‘Top Level’, no matter how deep the level of nesting.

is_page_downstream(63) will return true for all pages with id in (63, 65, 76, 78, 67, 81, 84)

And it will return false for pages with id in (23, 27)

Notes & Credit

Please note this was tested on WordPress 3.3.1. Also, credit must go to this post on CSS Tricks for helping me put this post together.