Here is another example of a pretty popular and supported web based application that is based on PHP. I am finding more and more that PHP is turning out to be more than just a scripting language. With applications like Facebook, WordPress, and SugarCRM you have to start wondering if there is something there.
I was also reading some blogs this weekend about what people use to develop PHP based sites and I was shocked to see how popular Notepad++ was. I have been using Notepad++ for a long time now for my WordPress site and a couple of other sites I support.