functions.php

<?php 

// Détermine si la page "$id" a comme ancêtre la page "$ancestor_id"
function is_page_ancestor( $id, $ancestor_id ) {
	$p = get_post( $id );
	while ( $p->post_parent ) {
		if ( $p->post_parent == $ancestor_id ) { return true; }
		$p = get_post( $p->post_parent );
	}
}

// Retourne le plus "haut" parent de la page "$id"
function get_page_highest_ancestor( $id ) {
	$p = get_post( $id );
	while ( $p->post_parent ) {
		$p = get_post( $p->post_parent );
	}
	return $p->ID;
}