[eluser]CtheB[/eluser]
Server: PHP5
Writing code in: PHP5
FC/BC: FC (forward-compatible to PHP6)
WHY: Because if in the future we update to PHP6 we don't have to change a lot of our code.
My personal opinion is: Start making all code in codeigniter php6 compatible and drop php4.
Why update to php6 in the future: Because of the improvements and because of Unicode.