Sounds like you are misunderstanding the point of what mod_rewrite does (which is REALLY common). It is intended to take a request for one URI and serve some resource from your actual server.
In your case if Apache receives a request to the server for /author/foobar.php, it would attempt serve up the file /author/foobar.nophp. I'm assuming this isn't actually a file on your server. Assuming you want to achieve the common task of URL's on your site not having file extensions, then you need to flip your view of how it works. At it's most basic you would have something like...
This will then take requests for /author/foobar and serve the file located at /author/foobar.php, therefore you must then change all URLs on your site to point to the extension less path.
RewriteRule ^/author/(.*)/?$ /author/$1.php
NB. If you have this code in your .htaccess file, not in a Vhost file, then the very first forward slash doesn't want to be there.