$adiff = json_decode('{"28":0,"29":0,"30":0,"31":0,"32":0,"33":0,"34":0,"35":0}',true);
$diff = json_decode('{"28":1,"29":1,"30":1,"31":1,"32":1,"33":1,"34":1,"35":1}',true);
$adiff = array_merge_recursive($diff,$adiff);
var_dump($adiff);

理论上应该返回：

array(16) { [28]=> int(1) [29]=> int(1) [20]=> int(1) [31]=> int(1) [32]=> int(1) [33]=> int(1) [34]=> int(1) [35]=> int(1) [36]=> int(0) [37]=> int(0) [38]=> int(0) [39]=> int(0) [40]=> int(0) [41]=> int(0) [42]=> int(0) [43]=> int(0) }

实际返回：

array(16) { [0]=> int(1) [1]=> int(1) [2]=> int(1) [3]=> int(1) [4]=> int(1) [5]=> int(1) [6]=> int(1) [7]=> int(1) [8]=> int(0) [9]=> int(0) [10]=> int(0) [11]=> int(0) [12]=> int(0) [13]=> int(0) [14]=> int(0) [15]=> int(0) }

这算不算一个bug？

更新：
这个不算bug,只是手册里面没有对数字键进行merge时会重置说明清楚。