Unfortunately that link is not useful. That link is not about building a PAR sensor, instead it's about connecting an existing PAR sensor to an Arduino.
You know, maybe not having a "proper answer" after five years simple means there is no answer? I've been looking into building a PAR sensor based on an Arduino as well. Without a working light spectrum analyser (I have yet to find one that is not the size of a piece of furniture and for lab use only) this is just not possible.
Point in case: the SQ-225 sensor mentioned in your link only works well under T5 fluorescent light... which imho makes it a terrible waste of money because if you know the spectrum of the light, all you need to do is measure the overall brightness and you can calculate the PAR!