Apache Reference: mod_setenvif

mod_setenvif - Conditional Environment Variable Creation
Since Apache 1.2, src/modules/standard/mod_setenvif.c
Alexei Kosut, Paul Sutton (1996)

mod_setenvif is a more advanced module for setting SSI/CGI environment variables. It sets variables depending on various client information found in the HTTP request. This ability is useful when combined with a special variable-based feature of the core module's deny and allow directives. In addition, some internal HTTP protocol behaviors of Apache can be controlled through various variables that are usually set with the SetEnvIf directive of this module.

Directives:

BrowserMatch    BrowserMatchNoCase
SetEnvIf    SetEnvIfNoCase