$ sudo apt-get update
$ sudo apt-get install php-xml-rss 

php-xml-rss is installed in your system.

Make ensure the php-xml-rss package were installed using the commands given below,

$ sudo dpkg-query -l | grep php-xml-rss *

You will get with php-xml-rss package name, version, architecture and description in a table

php-xml-rss package basic information: