I've used 3 shared hosting providers, all of which provide SSH access. And when I ssh in, and type "ps", I see my own processes, not the processes of anybody else. They are running as a different user.
You can have unique SSH users and still have all the PHP code run under a shared user on the web server. The machine you SSH in is most likely not the same as the one running your PHP code even.
https://news.ycombinator.com/item?id=29804968
I've used 3 shared hosting providers, all of which provide SSH access. And when I ssh in, and type "ps", I see my own processes, not the processes of anybody else. They are running as a different user.