Answer by StephanieQ for PHP not parsing wordpress rss feed

I know this is an old question but I came across it trying to fix my own WordPress RSS feed issue. In my case I was missing the PHP XML extension.

So I ranapt-get install php7.0-xml (I'm running PHP v7.0 on Ubuntu, check your php version), restarted my server and it totally fixed my problem!

For CentOS / Fedora / Red Hat on PHP 7:

yum install php70w-xml

Hope this helps somebody!