Why is my directory opening as http://mydomain.com instead of http://www.mydomain.com when i type mydomain.com in the browser? I have to say that in my admin settings the url is set to http://www.mydomain.com/.
Do you have a .htaccess file in root? it could be 301 redirecting 'www' to 'non www' and overriding admin settings.
That sounds correct to me. Try temporarily removing your htaccess file and see if that fixes the problem. If yes, you probably will want to take a look at the htaccess file itself or make a support ticket with phpLD.