GTK 2 has a lot of new widget classes. Most of the new classes do something better or a different way than an old class; and so the old widgets were deprecated in favor of the new one.
To keep backward compatibility (BC), the old widgets can still be used, but are likely to be removed in GTK 3 or so. That means that you don't have to rewrite your old programs totally, but still can use them without too much changes. However, you really shouldn't use any of the deprecated widgets in newly written code. Don't do it.
List of deprecated widgets
List of Deprecated Methods
The list may not be complete. If you find a deprecated widget that is not listed here, please write to php-gtk-doc.