The following command
wget --convert-links --backup-converted --html-extension --mirror http.//localhost/index.php
downloads index.php.html and back it up as index.php.html.orig before converting the links. When re-mirroring, wget looks for local index.php.orig which doesn't exist and thus re-download index.php.
Also, if there is no conversion needed, the local file is index.php.html, but wget looks for index.php.orig or index.php which do not exist.
I'm using wget version 1.9.1.
Nicolas