storage of session and global variables

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Can anyone tell enlighten me on the following:

where on the server are the session variables kept?

where on the server are global and local variables kept?

I would like to know this because I'm wondering that: if session variables reside in the server's RAM, then for a page that accesses a session variable more than once within its code, would it make more sense to reduce the strain of repeated access to a server's RAM by use of a global variable assigned to the value of the session variable, in which use of the global variable is employed instead of where there would have been instances of the session variable within the body of the code?

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So you are asking if it's faster to access a global var than a server var?

I don't think there is a difference as far as PHP is conserned.

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No, I'm not asking whether it's any faster to access either type of variable (although I'm pretty sure that any stored in RAM would result in a faster performance in any case), rather I'm asking are session variables stored in the server's RAM, and are global variables stored on the servers disk?

And, if session variables are stored in server RAM, I'm also asking if there's a danger of poorer server performance if its RAM were subjected to repeated access of the same session variable (and many clients could be trying to que up to do this) instead of repeated access to a global variable. I suppose in my original question, also suggested a measure to reduce the strain on a server's RAM if it were true that session variables are stored in server RAM.

I must apologise for the clumsy phrasing of the original question. I hope the re-phrased version above is clearer.

Regards.

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When a script starts, the session vars are read from disk and stored in RAM.
When the script finishes, the session vars are written to disk.

Global vars are never written to disk.

While the script runs, both the session vars and the global vars are stored in The computers working memory: RAM.

Thus, there is NO difference between the serverload for reading a session var and reading a global var.

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Thanks, thats very clear.