apache 2.2 - mod_rewrite: redirect from subdomain to main domain

I have two domains - domain.com and forum.domain.com that
points to the same directory.

I'd like redirect
all request from forum.domain.com to domain.com (for example: forum.domain.com/foo to
domain.com/forum/foo) without changing address in addres bar (hidden
redirect).

I wrote something like this and put
it into .htaccess
file:

Options
            +FollowSymlinks
RewriteEngine on 

RewriteCond
            %{HTTP_HOST} ^forum\.example\.net$
RewriteRule (.*)
            http://example.com/forum/$1 [L]

RewriteCond %{REQUEST_FILENAME} !-s
            [NC]
RewriteCond %{REQUEST_FILENAME} !-d [NC]
RewriteRule ^(.+)
            index.php/$1
            [L]

That
works only if I add Redirect
directive:

RewriteRule (.*)
            http://example.com/forum/$1
            [R,L]

But it changes
previous address in address
bar.

EDIT:

Ok,
let's make it simple. I added those two lines at the end of the
c:\windows\system32\drivers\etc\hosts on my local
computer:

127.0.0.3
            foo.net
127.0.0.3
            forum.foo.net

Now, I
created two virtual
hosts:

            foo.net:80>

 ServerAdmin admin@admin.com
 ServerName
            foo.net
DocumentRoot
            "C:/usr/src/foo"


            forum.foo.net:80>
 ServerAdmin admin@admin.com
 ServerName
            forum.foo.net
DocumentRoot
            "C:/usr/src/foo"

..and
directory called "foo", where i put two files: .htaccess and
index.php.

Index.php:

echo
            $_SERVER['PATH_INFO'];
?>

.htaccess:

Options
            +FollowSymlinks
RewriteEngine on
RewriteBase
            /

RewriteCond %{HTTP_HOST} ^forum\.foo\.net$
RewriteCond
            %{REQUEST_URI} !^/forum/

RewriteCond %{REQUEST_FILENAME} !-s
            [NC]
RewriteCond %{REQUEST_FILENAME} !-d [NC]
RewriteRule ^(.+)$
            /index.php/forum/$1 [L]

RewriteCond %{HTTP_HOST}
            !^forum\.foo\.net$
RewriteCond %{REQUEST_FILENAME} !-s
            [NC]
RewriteCond %{REQUEST_FILENAME} !-d [NC]
RewriteRule ^(.+)
            index.php/$1
            [L]

When I
type address http://forum.foo.net/test in address bar, it
displays /forum/test which is good. href="http://foo.net/a/b/c" rel="nofollow noreferrer">http://foo.net/a/b/c
shows /a/b/c which is good. But!
http://forum.foo.net/ displays empty value (should
display /forum).