How to add PHP code in post/page?

I am trying to use php code from rss feed reader to my page/post, but my post/ pages are not accepting it. I am able to add only html/javascript code into my page.

How do i add php code in post/page ?

I am trying to add php rss feed code in my post/page from the follwoing feed reader
http://rssfeedreader.com/

I appreciate your help. I already tried Exec php, php inline plugins but it does not work.

Thanks,