php-jsonencoded字符串，显示对象格式

我有一个结构类似-的数组（用print_r语句检查）

Array
(
    [0] => Array
        (
            [ads] => 343443543
            [ad_name] => Cup
            [cup_id] => 8869327
            [web_url] => www.yahoo.com
            [title] => Array
                (
                    [0] => title1
                    [1] => title2
                    [2] => title3
                    [3] => title4
                    [4] => title5
                )

            [description] => Array
                (
                    [0] => desc1
                    [1] => desc2
                    [2] => desc3
                )

            [new_title] => new_title1
        )

    [1] => Array
        (
            [ads] => 2325334
            [ad_name] => Cup
            [cup_id] => 8869327
            [web_url] => www.google.com
            [title] => Array
                (
                    [0] => title1
                    [1] => title2
                    [2] => title3
                    [3] => title4
                    [4] => title5
                )

            [description] => Array
                (
                    [0] => desc1
                    [1] => desc2
                    [2] => desc3
                )

            [new_title] => new_title1
        )
)

当我使用json_encode时，它返回的对象格式如下-

{"0":{"ads":"343443543","ad_name":"Cup","cup_id":"8869327","web_url":"www.yahoo.com","title":["title1","title2","title3","title4","title5"],"description":["desc1","desc2","desc3"],"new_title":"new_title1"},"1":{"ads":"2325334","ad_name":"Cup2","cup_id":"8869327","web_url":"www.google.com","title":["title1","title2","title3","title4","title5"],"description":["desc1","desc2","desc3"],"new_title":"new_title2"}

相反，我希望json字符串-

[{"ads":"343443543","ad_name":"Cup","cup_id":"8869327","web_url":"www.yahoo.com","title":["title1","title2","title3","title4","title5"],"description":["desc1","desc2","desc3"],"new_title":"new_title1"},{"ads":"2325334","ad_name":"Cup2","cup_id":"8869327","web_url":"www.google.com","title":["title1","title2","title3","title4","title5"],"description":["desc1","desc2","desc3"],"new_title":"new_title2"}]

我有另一个数组变量，它显示了正确的json格式。现在，确定这里出了什么问题。

感谢任何帮助。