将数组转换为UTF-8？ PHP JSON

i have multidimensional arrays generated by PHP with data from database ,but i have chars like "č ć š đ ž" and when i try to output that in json he just returns null , i did some reading about that ,and it says that JSON is only working with UTF-8. So how can i convert those arrays in UTF-8 ? but i still need arrays at the and?i have multidimensional arrays generated by PHP