Dev-PHP is a well-featured IDE for PHP. Fast and powerful, it is easy to use and has a lot of cool features : class browser, scripts preview, integration with PHP parsers, PHP-GTK library, and Xdebug of course. Dev-PHP-usb slips even into your pocket.383 weekly downloads
Phpyke is a full-featured IDE primarily aimed for PHP development, but supporting other major languages as well. It is created using Qt toolkit in C++ with editor based on QScintilla.