**
 * Check whether we are on a subpage
 *
 * @return mixed ID of the parent post or false if this is not a subpage.
 */
function wpdocs_is_subpage() {
    // Load details about this page.
    $post = get_post();
 
    // test to see if the page has a parent
    if ( is_page() && $post->post_parent ) {
        // Return the ID of the parent post.
       return $post->post_parent;
    // There is no parent so ...
    } else {
        // ... The answer to the question is false
          return false;
    }
   
}