I have to admit a bias towards using Java on the server side because I have so very much time invested in mastering the J2EE stack. That said, I will be using PHP for a major task for my customer in India because an existing open source project written in PHP gets us about 90% of our requirements ‘out of the box’.
I suppose that this is a common enough occurence for consultants: to get a job done efficiently often platform choice is not as important as the availability of open source projects that match project requirements.
For my own research and product development (which unfortunately has been put on hold – I am working on a full time job until near to the end of this year) I have a common pattern: some back end process that I would like to make available through a web interface and/or a SOAP/REST/XML-RPC interface. Much of my research programming is done in Common Lisp because it is a very productive language (I also like to use Python and Smalltalk for research programming). I am a fast coder, so in the past I would prototype in a language like Lisp and then quickly re-code in Java (or whatever other language I wanted to deliver with). With faster servers, I now favor just leaving working code in whatever languages are best for specific development and to live with the overhead of SOAP (etc.) interfaces.