how to test for all children (including multilevel grand-childern) of page

Question

I want to add an image to a parent page and all it's children including 2nd level grandchildren. I don't want the image on any other page.

here's what I have so far:

if(is_page() || is_single()) {
global $post;
if ($post->post_parent == '27' || $post->ID == '27') { 
    $ntest='<div class="clearlogo"><img alt="nodal Clear logo" width="150" src="'.get_stylesheet_directory_uri().'/images/nodal-clear-logo.png"></div>';
}else{ 
    $ntest='';
}

This works for 1st level and the parent, but not grandchildren. I thought about adding the 2nd level page ids, but thought there has be be a better way.

I feel like I'm missing something easy.

I'm writing this in a function that will add it to the content with the_content (full content below):

function rt_before_after($content) {
    if(is_page() || is_single()) {
      global $post;
      if ($post->post_parent == '27' || $post->ID == '27') { 
        $ntest='<div class="clearlogo"><img alt="nodal Clear logo" width="150" src="'.get_stylesheet_directory_uri().'/images/nodal-clear-logo.png"></div>';
      }else{ 
        $ntest='';
      }
        $beforecontent = $ntest.'<h1 class="title">'.get_the_title().'</h1>';

        $fullcontent = $beforecontent . $content;
    } else {
        $fullcontent = $content;
    }

    return $fullcontent;
}
add_filter('the_content', 'rt_before_after');

Answer 1

You can use get_ancestors() to check if the post have an ancestor parent id or grandparent id etc..

But you would have to check every post for this...

function rt_before_after($content) {
    if(is_page() || is_single()) {
      global $post;
      $ancestors = get_ancestors( $post->ID, 'page' );
      if ( in_array( 27, $ancestors ) || $post->ID == '27') {
        $ntest='<div class="clearlogo"><img alt="nodal Clear logo" width="150" src="'.get_stylesheet_directory_uri().'/images/nodal-clear-logo.png"></div>';
      }else{
        $ntest='';
      }
        $beforecontent = $ntest.'<h1 class="title">'.get_the_title().'</h1>';

        $fullcontent = $beforecontent . $content;
    } else {
        $fullcontent = $content;
    }

    return $fullcontent;
}
add_filter('the_content', 'rt_before_after');

You can go the other way around from the parent to children with get_pages()

And with this approach you can move it to seperate function and call it once and even cache it if you want. I would go for the second approach.

function rt_before_after($content) {
    if(is_page() || is_single()) {
      global $post;

      // get all the child pages and grandchild etc..
      $pages = get_pages( ['child_of'=>27] );
      $pages_ids = array_map( function( $item ) {
        return $item->ID;
      }, $pages );

      if ( in_array( $post->ID, $pages_ids ) || $post->ID == '27') {
        $ntest='<div class="clearlogo"><img alt="nodal Clear logo" width="150" src="'.get_stylesheet_directory_uri().'/images/nodal-clear-logo.png"></div>';
      }else{
        $ntest='';
      }
        $beforecontent = $ntest.'<h1 class="title">'.get_the_title().'</h1>';

        $fullcontent = $beforecontent . $content;
    } else {
        $fullcontent = $content;
    }

    return $fullcontent;
}
add_filter('the_content', 'rt_before_after');