By default, sessions have nothing at all inside of them. You can use the $_SESSION array to assign values to a session though.
So, you could go, $_SESSION['blah'] = 5, and that would put this into your session file:
blah|s:1:"5";
Any other variables that are registered to that session are split up similarly. So, if I also went $_SESSION['var'] = "Something", then the session file would now look like this:
blah|s:1:"5";var|s:9:"Something";