That looks like a URL string, not a JSON. If you want to make a JSON out of it, you first need to URL decode it, then use that data.
I don’t know if Yii includes a class to decode it, but if it doesn’t you can try the urldecode function.
It will give you something like
Order[item_list][]=5&Order[item_list][]=6
With that string, you can do whatever you want. If you wanted to decode a JSON, I guess you wanted to make an array out of it. In which case, you can use the explode function, and separate it with the "&" character, resulting in something like