I think you're misunderstanding at least one of those. [font=monospace]REMOTE_ADDR[/font] will show the user's IP, but [man]passthru/man executes a command locally (on your server).
If that's what you're trying to do, why would you want to give your system info to the user?
If, on the other hand, you're trying to pass a system command to the user's computer, what makes you think that's even remotely acceptable?