making subdomains out of a url?

i have this url

http://www.example.com/user.php?user=solomon

i want to convert this into

http://solomon.example.com

how can i do this!! thanks :))

p.s. im using plesk

Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^http://domain\.com
RewriteRule ^user.php http://domain1.com [R=301, L]