Browser/Client headers - which one's are standard / miximum?

[Randell D.]

I know not all browsers provide the same information thru headers when they
visit a server, however is there a standard that I can rely on which
provides a minimum set?

For example, am I guaranteed that I will receive the following, even if I
cannot guarantee their values from one client browser to another?

_SERVER["HTTP_ACCEPT"]
_SERVER["HTTP_ACCEPT_LANGUAGE"]
_SERVER["REMOTE_ADDR"]
_SERVER["HTTP_USER_AGENT"]

I want to create an md5 hash key and make it almost unique using a string of
text hard coded in my php script in addition to some variables value that
originated from the client that never changes during the clients session - I
want to use the hash as a basic form of sanity/security check (yes, I know
that packets can be sniffed from the network so its not meant to be fool
proof).

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