On the web, PHP/MySQL/AJAX is the top dog. Why? Because they are used to code the web, so therefore are popular on... the web. To be perfectly honest PHP is overhyped. Way overhyped. It's popular, but for very little reason other than the PHP team threw tons and tons of functions in the standard distribution for developers to use so they didn't have to code it themselves. It doesn't make the language better.
Check the Tiobe index: Tiobe Index PHP is NOT the most popular language. Every enterprise that I have worked with or consulted for that has a budget of $100,000+ uses Java primarily, possibly .Net, and maybe even Perl. But almost never PHP. PHP is generally most popular among webmasters who are working on tight budgets and small teams or individually. That's because PHP is very easy to learn and you can do most functions that websites require. However, for advanced technologies, PHP is not adequate. If you talk to technical architects who design network applications, they will almost never use PHP. These are the guys who usually have university degrees in computer science and have worked solely in computer science for a decade or more.
People like AOL, MSN, and Yahoo use ASP and not PHP. Microsoft does its own deal and makes its own stuff like popfly. Those the kinds of top companies?
Java Application Servers were the rage about 5 years ago. I am a consultant for a Microsoft Global Partner, but worked for a Fortune 500 company that did all of their work using Java and J2EE. Well, the company started a .NET line of business a couple years ago because their Java business is souring (this is a 5B+ a year company). State and Local governments are moving away from Oracle and Java to the .NET server platform and programming tools. I RARELY see successful startups using Java or J2EE as their platform of choice.