If you’re having problems with WHMCS’s Twitter integration feature on a CentOS/RHEL server, then chances are that you need to install the php-xml RPM.
The Twitter integration in WHMCS uses jQuery to make a POST request to announcements.php detailing the number of tweets to retrieve. This then in turn connects to the Twitter API, parses the response and returns it to the browser as HTML.
Something in this script requires one of the extensions provided by the php-xml RPM (dom, wddx, xmlreader, xmlwriter and xsl). If the module isn’t present, then the script fails silently and returns no tweets. You still get some HTML though – the Twitter icon, a single hyphen and the “Follow us on Twitter” button.