Back in the bad old, days, PHP had this thing called register_globals which would take any values from $GET or $POST (and other superglobals) and automatically turn these values into global variables in your script. E.g., if you supply a $POST["some_var"] to a script, the register_globals functionality would automatically define $some_var in your script. While the motivation behind this is ostensibly convenience, it results in big security problems and so this behavior has been gradually and gracefully (and wisely) deprecated. You might be able to imagine what would happen if someone were supply a value of 1 for $POST["user_is_administrator"] or something.
I don't recall why that warning would be shown, but suspect it has something to do with those bad old register_globals behaviors and their relation to sessions.
At any rate, that warning you are seeing can be turned off by setting session.bug_compat_42=Off in the appropriate PHP.ini file.
