modx function

function getlastid() {
    $q=$modx->prepare("SELECT `id` from `modx_site_content` order by `id` DESC limit 1");
	$q->execute();
	$lastid = $q->fetchAll(PDO::FETCH_ASSOC);
	return $lastid[0]['id'];
}

echo getlastid();

Fatal error: Call to a member function prepare() on a non-object in /var/www/firstopt/data/www/firstopt.ru/core/cache/includes/elements/modsnippet/53.include.cache.php on line 11

$q=$modx->prepare("SELECT `id` from `modx_site_content` order by `id` DESC limit 1");
	$q->execute();
	$lastid = $q->fetchAll(PDO::FETCH_ASSOC);
print_r($lastid);