array_shift();

$json = '{"20151224084":{"number":"9,7,6,8,0","dateline":"2015-12-24 20:02:55"},"20151224083":{"number":"4,2,7,5,1","dateline":"2015-12-24 19:52:55"},"20151224082":{"number":"6,5,5,5,7","dateline":"2015-12-24 19:43:36"},"20151224081":{"number":"3,8,6,0,8","dateline":"2015-12-24 19:33:01"},"20151224080":{"number":"5,1,6,7,9","dateline":"2015-12-24 19:23:01"},"20151224079":{"number":"3,8,0,3,5","dateline":"2015-12-24 19:13:05"},"20151224078":{"number":"1,2,2,8,4","dateline":"2015-12-24 19:02:57"},"20151224077":{"number":"1,4,9,1,1","dateline":"2015-12-24 18:52:58"},"20151224076":{"number":"3,2,4,0,3","dateline":"2015-12-24 18:43:25"},"20151224075":{"number":"0,3,2,9,7","dateline":"2015-12-24 18:32:59"}}'; $data = json_decode($json,true); // 方法1 $want = $data['20151224084']; // 方法2 $want = array_pop(array_reverse($data,true)); // 最笨的办法 $want = []; foreach($data as $key=>$value){ $want = [$key=>$value]; break; }

说明一下：第二种方式有个坑，就是当数组的长度特别长时，比如1000、10000，数组的翻转是一件很消耗性能的操作，所以，根据你的实际情况借鉴性的使用，第三种方式虽然笨，但是性能不错。
经友情提醒，刚才忘记了array_shift

json_decode($string, true)
转为数组后你肯定会了。