This morning, the Eclipse Foundation announced the release of the PHP Development Tools 2.0. PDT includes all the stuff you'd expect a GUI IDE to offer: syntax highlighting, refactorings, tools for tracking variables and classes. According to the Eclipse Foundation, 2.0 adds the following:

To support the object-oriented features of PHP, PDT 2.0 now includes:
    •    Type Hierarchy view that navigates object-oriented PHP code faster and more easily.
    •    Type and method navigation that allows for easy searching of PHP code based on type information.
    •    Override indicators that visually tag PHP methods that have been overridden.

Usability improvements to PDT 2.0 include:
    •    A new indexing and caching engine, based on the Eclipse Dynamic Language Toolkit (DLTK), which significantly improves the overall performance of common PDT operations.
    •    A new Mark Occurrences indicator that make it easier for developers to see where an element is referenced.
    •    More sophisticated Code Assist feature that is smarter about providing code completion options based on PHP variable types.

 This is a joint project headed up by Zend Technologies. As the big name in PHP, I'd expect they'll remain on top of this dev environment and continue to add cool new features.