Why do you want to do this? Depending on your site, it might not be possible to translate all of it to Wordpress. Wordpress is a CMS and RoR is a framework.
You might be able to reuse the CSS but that’s about it.
Personally, I’d stick with RoR over PHP in general unless there’s some feature of PHP you need.
Working on the site for a friend who already had it build with RoR but now wants it with WordPress. Theres no real reason to use PHP. I think he wanted to use WordPress because thats more friendly to someone without any experience.
My apologies if that sounded like a stupid question.