Rewrite Rule for subdomains?

Can someone help me with a Rewrite Rule?

My document root is: /srv/www/htdocs/www.mydomain.com/htdocs

How can I make a rule that changes the webadress www.sub.mydomain.com to
www.mydomain.com/Page.php?URL=sub

<Directory /srv/www/htdocs/www.mydomain.com/htdocs>
RewriteEngine On
RewriteRule [WHAT DO I PUT HERE?]
</Directory>

Regards from Denmark, hey it's snowing here!

Dan