I would ask how did you get it encoded? I would guess you wrote some code to encoded it originally, so where is that code? I would hope you keep a backup of everything you do.
Also just looking it over, it appears to be content that was to be displayed I would think there would be an accompanying piece of script that was to decode it when that content was requested. I don't know how it was all setup to work but I am guessing another php file included it and that file may have had the decoding script or it included another php file and it had the decoding script.
In either case is sounds like you are somehow missing at least 2 critical files for this website you made. :bemused:
Best of luck, I don't recognize it, and I would guess its a real shot in the dark as to what encoding was used. Maybe someone here will recognize the format but I wouldn't be surprised if a few different types of encoding are similar enough that even a good guess may come up wrong.