4.4.6 Defining environment variables
You can define environment variables as per client request. You can define environment variables and cancel the settings of environment variables using the host name and IP address of the client, who sends the request.
SetEnvIfNoCase Request_URI "\.(gif|jpg)$" request_is_image
In such cases, if the file extension is .gif or .jpg (this directive is not case sensitive), the environment variable request_is_image is passed to CGI programs.