You can modify .htaccess file under your domain mapping root to setup redirection.
To redirect www.domain.com to www.newdomain.com , add following line to .htaccess:
RedirectMatch 301 ^(.*)$ http://www.newdomain.com
To redirect domain.com to www.domain.com, add following lines to .htaccess:
RewriteCond %{HTTP_HOST} ^domain\.com
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=permanent,L]
To redirect domain.com to domain.com/hello.php, add following lines to .htaccess:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain.com$
RewriteRule ^$ http://domain.com/hello.php [L,R=301]