Cache JSON feed round an existing number then add it to the json file that's cached

I need to use my existing php file that caches my json files for use on my website. I need to get a certain value from the json that's being processed and round the decimal up and also down to the nearest whole number. I need those two values then added to the end of the cached json file along with all the other information that's existing. I am not sure if this is even possible if you think this is something that you could do please let me know. I have the php file and can provide an example file for you. I must use the existing file as it's already fully implemented in my site.