I have taken a look at this DB wrapper class and it seems to work OK apart from on the first example given in your example file.
With the input set as $config defining an array, I always seem to get the error of: DB: Classname isn't set! on line 56. I can see that this error is displayed when the $config['class'] is not set.
Within the example file I have this set as mysql, but it doesnt seem to register, I have tried to display this array item within the error message but nothing extra is displayed (or the array item may be blank).
Within the example we can always use the class example to achieve the same result if we want to avoid JSON data, but I thought I had better let you know that this problem is apparent.