I had to add another answer here because I didn't really want to go through the steps in the other solutions. It is extremely simple and requires no extensions, includes etc and is what I prefer. It's very easy and very fast.
First just json_encode the variable in question:
echo json_encode($theResult);
Copy the result you get into the JSON Editor at http://jsoneditoronline.org/ just copy it into the left side pane, click Copy > and it pretty prints the JSON in a really nice tree format.
To each their own, but hopefully this helps some others have one more nice option! :)