$keys = array("category" => 1);
$initial = array("items" => array());
$reduce = "function (obj, prev) { prev.items.push(obj.name); }";
$g = $collection->group($keys, $initial, $reduce);
print_r($g);   //结果如下。
Array
(
    [retval] => Array
        (
            [0] => Array
                (
                    [category] => fruit
                    [items] => Array
                        (
                            [0] => apple
                            [1] => peach
                            [2] => banana
                        )
                )
            [1] => Array
                (
                    [category] => veggie
                    [items] => Array
                        (
                            [0] => corn
                            [1] => broccoli
                        )
                )
        )
    [count] => 5
    [keys] => 2
    [ok] => 1
)