RoR rocks. Check out our latest deployment of a Rails app -
www.eroslife.com. Runs on Amazon EC2 with clustering and rotating backups. No virtual hosting (in the traditional sense) and catastrophes to worry about anymore since everything runs on Amazon's backbone. Hit me up if you need advice/help on your rails app.
I'm a certified Java programmer with over 11 years exp, but if you're thinking of launching an adult site with social networking capabilities, go with RoR.
Now, back to your original question or RoR or Php. I personally don't know much php other than to hack it for my needs, but it seems like RoR and other 'next-gen frameworks' like Django are fast gaining momentum. With that said, php has a very very long track record and a number of high traffic sites already use php.
But RoR is great because the language is very versatile (great for DSL) and both Django and Rails appear more intuitive to understand than php for some developers. Anyway, I encourage you to give RoR a shot.