7.3. The URI Translation Phase
One of the web's virtues is its Uniform Resource Identifier (URI) and Uniform Resource Locator (URL) standards. End users never know for sure what is sitting behind a URI. It could be a static file, a dynamic script, a proxied request, or something even more esoteric. The file or program behind a URI may change over time, but this too is transparent to the end user.
Much of Apache's power and flexibility comes from its highly configurable URI translation phase, which comes relatively early in the request cycle, after the post_read_request and before the header_parser phases. During this phase, the URI requested by the remote browser is translated into a physical filename, which may in turn be returned directly to the browser as a static document or passed on to a CGI script or Apache API module for processing. During URI translation, each module that has declared its interest in handling this phase is given a chance to modify the URI. The first module to handle the phase (i.e., return something other than a status of DECLINED) terminates the phase. This prevents several URI translators from interfering with one another by trying to map the same URI onto several different file paths.
By default, two URI translation handlers are installed in stock Apache distributions. The mod_alias module looks for the existence of several directives that may apply to the current URI. These include Alias , ScriptAlias, Redirect, AliasMatch, and other directives. ...