On Ubuntu 12.10, try sudo apt-get install php5-xmlrpc
XML-RPC support in PHP is not enabled by default. You will need to use the --with-xmlrpc[=DIR] configuration option when compiling PHP to enable XML-RPC support. This extension is bundled into PHP as of 4.1.0.
Note: On Windows you just have to uncomment the following line in php.ini :-
If you run CentOS do yum install php5-xmlrpc and run /sbin/service httpd restart and you ready agin :)