RewriteEngine on
RewriteCond %{HTTP_HOST} ^www.
RewriteCond $1 !^(site\.php|assets|robots\.txt)
RewriteRule ^(.*)$ /site.php?/$1 [L]
RewriteCond %{HTTP_HOST} ^admin.
RewriteCond $1 !^(admin\.php|assets|robots\.txt)
RewriteRule ^(.*)$ /admin.php?/$1 [L]