This is a question I've been dealing with myself for some time. I've alot of professional experience with PHP but have done some work in Ruby (on Rails) too. I'm looking to build a decent size site and in the end have opted for PHP for the following reasons:

- familiarity with the language - wide support and hosted everywhere - not restricted by Rails 'norms' - easier to get up and running on a live server, even if it's your own virtual box - proven scalability - personally not mad on ActiveRecord (Rails way of interacting with DB). Would rather have greater control over my DB interactions, which I know is possible in Rails but it's very non-standard

Ruby IMHO is a far superior language to PHP, but you will have to learn a framework to get it up and running on the web, whether it's Rails, Sinatra or something else. Same with Python. PHP doesn't require a framework, although plenty exist, so it'll be easier to get an initial prototype up and running.