What do you think about predefining _post and _get variables in the beginning of your script? By predefining I mean copying the _post/_get variables into local ones. (Eg: $PST_username = $_POST['username'] )

The "pro" --  to make the script more "programmer-friendly".
The "con" -- copying and it's sideeffects [cpu/mem].

Does it really matter, or is it a question of code style?

Thanks for the reply.

Can you expand a bit on what makes it a security compromise?

p.s. register_globals = off

It dosn't matter if where you have it, just as long as you have it, I always put the "carried" varbs at the top, just a way to make sure that I have them, and don't for get....