[WordPress]functions.phpで固定ページの親ページ判定

functions.php

<h3>functions.php</h3>
function page_is_ancestor_of($slug){
    global $post;
    $page = get_page_by_path($slug);
    $result = false;
    if(isset($page)){
        foreach ($post->ancestors as $ancestor) {
        if($ancestor == $page->ID){ $result = true; }
        }
    }
    return $result;
}