RewriteEngine on

RewriteCond %{HTTP_HOST} ^(www.)?alt.domain.com$ [NC]
RewriteCond %{REQUEST_URI} !^/altklasor/

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^(.*)$ /altklasor/$1 [L]

RewriteCond %{HTTP_HOST} ^(www.)?alt.domain.com$ [NC]
RewriteRule ^(/)?$ altklasor/index.php [L]