Source

File: wp-includes/post.php

function get_page_hierarchy( &$pages, $page_id = 0 ) {
	if ( empty( $pages ) ) {
		return array();
	}

	$children = array();
	foreach ( (array) $pages as $p ) {
		$parent_id                = intval( $p->post_parent );
		$children[ $parent_id ][] = $p;
	}

	$result = array();
	_page_traverse_name( $page_id, $children, $result );

	return $result;
}