Reinhold Weber has posted his lists of some of the similarities and differences between Ruby and PHP:
If you are a PHP developer chances are you have heard some of the buzz on Ruby on Rails, an open source web framework for rapid application development. [...] These are similarities and differences of Ruby compared to PHP. If you know PHP, this should give you a good and quick insight in what Ruby is all about and how it compares to PHP.
Some of the similarities include the dynamic typing, class scoping, heredoc abilities and current object references. Differences include syntax differences, method calling, naming conditions and Ruby's lack of interface/abstract classes.