This would be a long pros and cons discussion. If you used any other programming language with a C/C++ like syntax you should choose PHP because you'll learn it faster.
If you use a framework like symfony, or cakePHP you will get a very similar end-result to ROR and it will work using apache and a normal php installation. The learning curve is probably about the same (shorter if you already know php).
Ruby on Rails vs PHP is an article I wrote a while back that goes into this topic some. Both technologies have advanced a little since I first wrote it, but the overall gist is the same. You can run Ruby itself as a web app or there are other plugins / gems to do it for you like the Camping gem. With a newer technology called Passenger, you can run Rails through Apache like PHP does instead of using Apache's mod proxy to a Mongrel cluster. I've been using Passenger for a few months now and really like it. Pro Spotlight (link in my sig) is running through Passenger.