奇怪的php乱码问题

我在用php从数据库查询内容时，如果在查询之前不加
mysql_query("set names 'gbk'");
这条语句时，中文乱码，
我用json传递数据，出现如下格式
[{"text":"?????","id":"-100","leaf":false,"cls":"folder"},{"text":"????1","id":"1","leaf":false,"cls":"folder"},{"text":"????2","id":"2","leaf":false,"cls":"folder"},{"text":"????3","id":"3","leaf":false,"cls":"folder"}]
如果加上
mysql_query("set names 'gbk'");
这条语句时，有中文的字段显示不出来，出现如下格式：[{"text":"","id":"-100","leaf":false,"cls":"folder"},{"text":"","id":"1","leaf":false,"cls":"folder"},{"text":"","id":"2","leaf":false,"cls":"folder"},{"text":"","id":"3","leaf":false,"cls":"folder"}]，
也就是刚才乱码的东西，现在显示不出来了，请各位大侠看看怎么回事？