getenv -- Get the value of an environment variable


string getenv(string varname);

Returns the value of the environment variable varname, or false on an error.

$ip = getenv("REMOTE_ADDR"); // get the ip number of the user 

You can see a list of all the environmental variables by using phpinfo(). You can find out what many of them mean by taking a look at the CGI specification, specifically the page on environmental variables.