ht access mod rewrite subdomain to variable

Hi,
I am looking for some help with a new site I am developing. What I need to do is have members to the site being able to have their own subdomain for each account: for example

account1.XXX.com
account2.XXX.com
account3.XXX.com

What I have been looking at is being able to use PHP to write the htaccess file and rewrite the URL to a URL with a variable with out actually creating the subdomains. I supose like creating a fake subdomain for each account holder.

Therefore I think the way to do it is with .htaccess to create something like if the user enters

account1.XXX.com  they get redirected to
account1.XXX.com/index.php/?a=1234 where alll the information for the account is pulled from a mysql database using the variable 1234

What I have no clue how to do and after searching the web I cannot work out how to do the subdomain rewrite of this.

Any help to get me going would be great

Thanks