Figured it out:
LOCK TABLE `categories` WRITE;
SELECT @myLeft := `lft` FROM `categories`
WHERE `id` = '. $where .';
UPDATE `categories` SET `rght` = `rght` + 2 WHERE `rght` > @myLeft;
UPDATE `categories` SET `lft` = `lft` + 2 WHERE `lft` > @myLeft OR `lft` = @myLeft;
INSERT INTO `categories`(`name`, `lft`, `rght`) VALUES("'. $name .'", @myLeft, @myLeft + 1);
UNLOCK TABLES;