Modify someone else's session register

Hi. Let's suppose I have 3 users in my site, each of them generating their respective session variables. Assuming I have a session variable named $variable, I'll end up in the server with 3 values: $variable_1, $variable_2 and $variable_3. My question is: is there a way I can change let's say the variable of the second user when the third user is running the php script? Can I access (and change) the session registers generated by the users? Thanks for your help, Alex.