Ruby on Rails is a framework, not a language like php. So it would be better to compare Ruby on Rails to something like the Zend Framework or other php frameworks.
With that being said. I find ROR a much faster development environment. I was asking the same question as you about a year ago and discovered the best thing to do is try different PHP frameworks and ROR. If you are coming from a PHP background, you'll find ROR a bit frustrating at times due to its concept of convention over configuration.
I will say that I have switched to ROR as my primary development target. And now things that used to take several days with php take a couple hours at most with ROR. I will definitely say that just by learning ROR, I became a better programmer.
An important thing to remember is PHP is supported by almost every host at a cheap rate, and ROR will require a VPS or dedicated host.
To sum it all up. I think ROR is better than any php framework.