As a Back-end Developer (as well as a Windows-based Application Developer), my focus has been primarily on developing skills in abstraction more so than specific languages - i.e. modeling the real world requirements into objects and patterns to create the best road to future scalability.
I do enjoy learning the specific features of a particular language - however focusing on the meta level has been my preference. In fact, I balked at learning PHP at first, preferring to wait until 5.0 was the dominant version so that coding OO-based solutions in the positions I've held had a lot less interference from the language.
My SQL experience includes a few different brands: Most of it is in MySQL - over 3 years. I have done a project using PostgreSQL. At Digital Insight, it was Informix, and when working at Media Services writing Accounting applications, it was Firebird.
I'd love to answer any questions you have on my experience, skillset, where I've been and why I prefer this framework or that Ajax library.