ini is a good type of file to save stuff it has an okay speed and easy to edit i think.
So yes you should probably try $readini and /writein
My advice regarding structure of the file would be to make seperate sections for each userwhere all his info, stats, progress would be saved.
So the file would look something like this
Just an example. You can put as many values as you want in each section.
I hope i helped! Good luck with your game!