keeping user images in the assets/uploads dir - multiprocess question

My system allows users to upload images. After each upload (to /assets/uploads) I also copy this file to my S3 bucket for this user. At the end of their system use and logoff, I delete all of the images in /assets/uploads since I have already copied their files to their s3 bucket. 
My system is multiuser. Does each user have their own copy of the CI environment on disk? I am asking if two users are sharing the same directory space in /asset/uploads. If that is so I really should NOT delete *.png from /assets/uploads? I do have the names of all of each users files in my table and could delete them one by one.  I guess I am confused since each php program (I think) is a separate process, unlike the way we used to program in multi threading in C where all the code was shared with the use of a state machine. In CI3 does each process have its own copy of the code on the hard drive?

proof that an old dog can learn new tricks