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/*-----------------------------------------------------------------------------------*/
/*  Megvizsgálja, hogy az adott oldal egy konkrét oldalnak a leszármazottja-e
/*-----------------------------------------------------------------------------------*/
function is_tree( $page_id_or_slug ) { // $page_id_or_slug = The ID of the page we're looking for pages underneath
    global $post; // load details about this page

    if ( !is_numeric( $page_id_or_slug ) ) { // Used this code to change a slug to an ID, but had to change is_int to is_numeric for it to work: http://bavotasan.com/2011/is_child-conditional-function-for-wordpress/
        $page = get_page_by_path( $page_id_or_slug );
        $page_id_or_slug = $page->ID;
    }

    if ( is_page() && ( $post->post_parent == $page_id_or_slug || (is_page( $page_id_or_slug ) || in_array($page_id_or_slug, $post->ancestors) ) ) )
        return true; // we're at the page or at a sub page
    else
        return false; // we're elsewhere
};

is_tree(19)