Wordpress: function returns true if current page is child of specified page

ml_is_child_of.php

<?php
function ml_is_child_of( $parent_id, $post_id = null, $post_type = 'page' )
{
	if ( !is_page() )
		return false;

	$post_id = ( $post_id ) ? $post_id : get_the_ID();
	$ancestors = get_ancestors( $post_id, $post_type );

	return in_array( $parent_id, $ancestors );
}