I build my website on PHP codeignitor framework. Can I use ruby on rail framework with that website. Should I have to use any cross platform?
Given that both frameworks can run side-by-side on the same server then I don't see a reason why not but there wouldn't be anyway to run both frameworks within the same files since for example .php can only execute PHP code.
I've never heard of this being done in practice, you're best to think of them as separate beasts and have an API between them. But I'd encourage you to stick with one platform unless there's a really good reason to split them.