Well, when you grab results from the database, prepared statements or otherwise, you’re storing those results either in an object or an array to process them. To the best of my knowledge there isn’t much (if any) difference in overhead between objects and arrays. Both are just storing that data in memory while you work with it, clearing it either when you unset the obj/array or when the script ends. So as far as performance goes, storing it in an array should be just fine.