RSS Icon Prompts to Save index.php file

I'm sorry if this has been covered before, but I couldn't find anything by searching the forum history.

Whenever I click the RSS icon on the site pages, it prompts me to Open or Save the index.php, rather than just taking me to the feed back. Even if I select "open", I am just returned the source code rather than seeing the actual page.

Is there a setting somewhere that toggles/changes this?