If you want to redirect a subdomain like sub.domain.tld into a subdirectory of the website and keep the original URL in the browser location bar, you may use the following apache directives.

RewriteEngine on
RewriteCond %{HTTP_HOST} ^sub.domain.tld [NC]
RewriteRule ^/(.*)$ /sub/$1 [L]

This rewrite rule can be added into a .htaccess file in the website root or inside the vhost file. If you use ISPConfig 2 or 3, you can add this also into the apache directives field in the website settings.

Replace sub.domain.tld with the subdomain that shall be redirected and /sub/ with the path to the directory were the pages for this subdomain are located.

If all your visitors shall access your website with a URL like www.domain.com and not without www, use the following apache rewrite rule for redirecting them.

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www
RewriteRule (.*) http://www.%{HTTP_HOST}$1 [L,R]

The apache rewrite rule can be added in a .htaccess file in the website root directory or if you use ISPConfig 2 or 3, you can also add the rwrite rule into the apache directives field of the website.