I'm not really a fan of big and magical frameworks. So the existence of Rails doesn't actually make me think Ruby is superior to PHP, although I need to try Sinatra. That being said I like Ruby's OOP. But in the end all OOP is the same. In my opinion, if you're an experienced PHP developer, and you're looking to expand your skillset, gaining another standard web development OOP language isn't going to change much of your value proposition. I would look closer into things like Node, Golang, Rust, Functional languages and Python (for scientific computing). Getting skills in Ruby in Rails seems like you're just adding another web dev language to your existing web dev language capability. It's all about MB vs MC.
This is not quite as in-depth an argument as one might have hoped for. If you really have so many reasons for the superiority of Rails over PHP then don't beg the question.
I mean, if this were serious, it would be "Why you should use Ruby over PHP" anyway, wouldn't it? If you want to compare frameworks, compare Rails and Laravel.
Interesting. I do feel as though Rails could replace PHP as a mindshare as time goes on.