This problem is solved by having mod_include implement only the bare minimum
of SSI tags. Mod_include also implements a hook to allow other modules to
extend its abilities. For example, in Apache 2.0 mod_include does not
implement the perl tag. If mod_perl wishes to implement this tag, then it
is free to do so. In the near future, mod_include will also stop implementing
the CGI tag, as that tag will move to mod_cgi and mod_cgid. This will remove
the possibility that a site that doesn’t want CGI scripts will accidentally
enable them with Server Side Includes. If mod_cgi is not included in the
server, then all CGI abilities will be removed.
Modules are able to extend mod_include’s abilities by calling
ap_register_include_handler. This function accepts a character string, which
represents the tag that mod_include is looking for, and a function that
implements the tag. Mod_include reads the input passed to it, and parses it
searching for the “