json_encode 怎么把空数据转换成无显示

[liujianliang]

{"step":"1","stepName":"\u7533\u8bf7","preStep":[null],"nextStep":["2"],"desc":""}
当某个元素无值时就会显示为null
因为业务需要希望显示成这个格式
{"step":1,"stepName":"\u7533\u8bf7","preStep":[],"nextStep":[2],"desc":""}
就是数字不用字符串形式且null时不显示null
该怎么做？

[xuzuning]

$str = '{"step":"1","stepName":"\u7533\u8bf7","preStep":[null],"nextStep":["2"],"desc":""} ';

$p = json_decode($str);

echo str_replace('[null]', '[]', json_encode($p));