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.
Note: This function does not work in ISAPI mode.