a) That you don't define both methods of replacement (Simple & PHP)
b) That the PHP call back method is valid.
Now, I'm mainly looking at b) here (still trying to figure out how to setup checking the inputted data, but I'll get that soon). I have read and reread the add-on DataWriter and Controller. I'm at a loss here. Its probably something simple I'm missing again. (The last one was extremely simple, solved by Shadab.)