This article looks at some of the strengths and weaknesses of the different options. You need to consider Javascript solutions like JQuery as well. In part, it depends on what you're going to do with it. Mobile apps are becoming a bigger part of the picture but that may not be part of what you want to do. How well PHP integrates with these different approaches is then another question once you establish your preference for a particular approach based on the other criteria.
http://www.codeproject.com/KB/HTML/Enterprise-RIA.aspx
Note: I can say from personal experience that it is quite easy to implement JQuery plugins and requires very little startup time or learning. You don't need to know anything about Javascript to use them. My perception is that you would have to invest some time and energy (and maybe $) if you wanted to start using Flex or Silverlight. It depends how serious you are about it and whether you'll get sufficient payback for that investment. If you are already a Flash or .Net developer, then that would be another factor in the decision.
