目录混乱友好的URL（htaccess）由PHP处理

I'm new to friendly urls so maybe something basic is missing.

My htaccess contains:

 RewriteRule ^([a-zA-Z0-9-/+]+)/?$ /index.php?surf=$1

inside index.php I handle the request like this:

switch ($_GET['surf']) {
    case "whatever":    
...

So links are like domain.com/home and I include a home.php using index.php for the purpose

Everything works for standard cases, but if someone inputs the name of a directory as parameter like: domain.com/css

The page gets messed up. The adress bar shows: http://domain.com/css/?surf=css and the content is like the default case in index.php (home), but without styles and such.

I guess it is trying to visit http://domain.com/css/index.php?surf=css after the htaccess and index stuff.

How can I fix this?

*Note that I have some index.php in some other directories that I would like to have access.

add these lines:

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([a-zA-Z0-9-/+]+)/?$ /index.php?surf=$1

This prevents redirecting real files and folders.