Via this new post on Nexen.net, there's information about an extension that's looking to provide another cross-platform solution to the creation of local GUI software with PHP - PHP-Qt.

PHP-Qt is an extension for PHP5 that aims to write software with the Qt Toolkit. It provides an object-oriented interface to the Qt4 Framework and allowes to write Qt applications in the PHP language.

Qt by Trolltech is a C++ toolkit for cross-platform GUI application development. Qt provides single-source portability across Microsoft Windows, Mac OS X, Linux, all major commercial Unix variants, and embedded Linux.

Their first (and latest) release, version 0.0.1, has been posted and is ready for some testing. You need to have PHP 5.1RC1 or later to get it working (as well as the Qt4 development files, of course), but it looks like it has some good potential...