If you really need this function, you can just extend the mysqli_result class with a function like this one.

<?php
        public function fetch_all($resulttype = MYSQLI_NUM)
        {
            if (method_exists('mysqli_result', 'fetch_all')) # Compatibility layer with PHP < 5.3
                $res = parent::fetch_all($resulttype);
            else
                for ($res = array(); $tmp = $this->fetch_array($resulttype);) $res[] = $tmp;

            return $res;
        }
?>