Rather than use words, let me let the graph below do the talking:
This graph comes from SitePoint's report, The State Of Web Development 2006/2007 -- 53 pages of "results, analysis and commentary on the state of Web Development in 2006/2007" based on a survey of 5,000 web developers. If you can't pony up the $795 single-user fee for the report, there's a free preview of the report as well as an article on the graph shown above.