PHP ErrorDocument and Apache Environment Vars

Is it possible to get at the variables that Apaches sets after an error? For example, I am setting the ErrorDocument for a 404 error to a php script. Now, in the script, I wish to refer to some of the Apache variables, at minimum REDIRECT_STATUS and REDIRECT_URL. I haven't been able to find them as environment vars, and phpinfo() doesn't know anything about them. Is there something that I've forgotten? It seems to me that this must be possible, I just can't figure out how!! Thanks, Ben