Return the ID of the parent page if the page has a parent. Just add this snippet to the functions.php of your wordpress theme and your ready to get the parent page ID.

[code lang=”php”]
function is_subpage() {
    global $post;
    if ( is_page() && $post->post_parent ) {
        return $post->post_parent;
    } else {
        return false;
    }
}
[/code]