if ((strpos($_SERVER['HTTP_HOST'],'www.')===false)) { header('Location: http://www.yourdomain.com'); exit(); }

http://www.bradino.com/php/force-www-subdomain/

Sometimes you do not want users to access your domain without the www. If you want to force www.yourdomain.com and redirect yourdomain.com to www.yourdomain.com here is a simple way to do it.

Force WWW Subdomain

PHP