xsend

I've been looking at a project where I'm going to use "private files" to make sure that only certain people can download certain files. I'd heard that this method was slower, since the web server can't serve the file directly. Instead it has to go through php.

mod_xsendfile

mod_xsendfile is a small Apache2 module that processes X-SENDFILE headers registered by the original output handler.

If it encounters the presence of such header it will discard all output and send the file specified by that header instead using Apache internals including all optimizations like caching-headers and sendfile or mmap if configured.

It is useful for processing script-output of e.g. php, perl or any cgi.
Referance from http://tn123.ath.cx/mod_xsendfile/