Redirect wildcard subdomains to subdirectory, without changing URL in address bar

 I've read a lot of questions and answers about this on here but none that seem to solve my specific problem.

I want to redirect any subdomain to the subdirectory to match.

So: x.domain.com would go to domain.com/x, and y.domain.com would go to domain.com/y - But I want to do this without the URL in the address bar changing.

This can be achieved in .htaccess without mod_proxy provided your server is configured to allow wildcard subdomains. (I achieved that in JustHost by creating a subomain manually named *). Add this to your .htaccess file:

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.website\.com$
RewriteCond %{HTTP_HOST} ^(\w+)\.website\.com$
RewriteCond %{REQUEST_URI}:%1 !^/([^/]+)/([^:]*):\1
RewriteRule ^(.*)$ /%1/$1 [QSA]