The mod_setenvifplus module allows you to set environment variables
according to whether different aspects of the request match regular
expressions you specify. These environment variables can be used by
other parts of the server to make decisions about actions to be taken,
e.g. by using mod_qos or to propagate supplemental information to the
appliaction via HTTP header data. mod_setenvifplus is derived from the
standard Apache modules mod_setenvif and mod_headers.

WWW: http://modsetenvifplus.sourceforge.net/