On the ThinkPHP blog today there's a new post looking at one of the better offerings for making desktop applications with the web-related skills you already know - CodeStrong (as a part of the Appcelerator/Titanium project).
Looking how to develop a desktop application in a short term, I came accross "Titanium Developer". This Open-Source tool helps you to create desktop apps. The special feature about it is the possibility to use well-known Web-Technologies such as HTML, JavaScript and PHP. Thus, learning a new programming language is no longer necessary. Another advantage is that you can directly create binaries for Mac, Linux and Windows using the same code.
They look at some example code to create a basic "Hello World" kind of tool and show how PHP can be embedded directly into the code, as made possible by the native PHP support introduced from contributions from people like Ben Ramsey.