The easiest way to learn any programming language is to create a project using that language. For Ruby, search for “Why the Lucky Stiff”. Thats the nickname of the author of one of the most user friendly Ruby books ever written. Its a comical introduction to Ruby. You will love it.
Visit rubyforge.org to get access to sample ruby code you can study for use in your own applications.
There is a small game-like tutor called Hackety Hack. It introduces you to Ruby in the same way that tutors used to teach Turtle Logo. Simple executable exercises.
Ruby is pretty similar to PHP. I do both and I can tell you there are fewer concepts you may have to discard since you are already familiar with PHP.