We can use .htaccess to redirect http://example.com to http://www.example.com by using the rewrite engine. But i find another simple way to implement the redirection just by setting up a VirtualHost.

Assume you have the current VirtualHost setting as follow.

<VirtualHost *:80>
  ServerName www.example.com
  DocumentRoot /var/www/<webroot>/
  <Directory /var/www/<webroot>/>
    Options Indexes FollowSymLinks MultiViews
    AllowOverride All
    Order allow,deny
    Allow from all
  </Directory>
</VirtualHost>

You could setup a new VirtualHost as follow to redirect example.com to http://www.example.com.

<VirtualHost *:80>
  ServerAlias example.com
  RedirectMatch permanent ^/(.*) http://www.example.com/$1
</VirtualHost>

Done =)

Reference: StackOverflow – apache redirect from non www to www