As mentioned in this official release on their site, the Appcelerator project has officially launched PHP support for their product as an extension.

Appcelerator®, the leading platform for rapidly developing mobile, desktop, and web applications with web technologies, announced its support for building desktop and netbook applications with PHP. Developers can now use PHP, JavaScript, Python, or Ruby to quickly build native PC, Macintosh, or Linux applications with the web skills they use today. [...] Seeing the opportunity for desktop and netbook app development, Titanium community member, Zend Certified Engineer, and software architect Ben Ramsey decided to lead the charge for integrating PHP, with Titanium.

You can grab the latest version of Appcelerator from their website and get coding right away. If you'd like to see some examples to get you started, check out this category on Ben's blog for helpful tips to get started.