For many reasons, a server can never allow access to its entire
directory hierarchy. Although there is really no indication of
this given to the web browser, every path given in a requested
URI is therefore a virtual path; early in the processing of a
request, the virtual path given in the request must be
translated to a path relative to the filesystem root, so that
Apache can determine what resource is really being requested.
This path can be considered to be a physical path, although it
may not physically exist. For instance, in mod_perl systems, you
may intend that the translated path does not physically exist,
because your module responds when it sees a request for this
non-existent path by sending a virtual document. It creates the
document on the fly, specifically for that request, and the
document then vanishes. Many of the documents you see on the Web
(for example, most documents that change their appearance
depending on what the browser asks for) do not physically exist.
This is one of the most impor