Need help updating youtube android app v2 to v3

I purchased a YouTube video app a few years ago i did some modifications to it but now that YouTube API has changed to v3, and my app doesn’t seen to work anymore, I don’t understand much PHP but i noticed i will need a API key and modify the PHP file who’s responsible to get the data from YouTube and i was wondering if somebody could help me out with this since the seller haven’t respond in more than a year.
here is the app i purchased