The PHP-GTK Community website points out the latest version of a tool GTK developers can use to plan out the screens for their applications - Glade v3.3.4 for Windows.

After giving this new version a spin, I can only suggest PHP-GTK2 developers already using Glade try it too, as the single-window UI improvement definitely changes impressions regarding the usability of Glade.

You can find out more about the Glade software from their website and specifically about this build from this (Italian) thread on the forumfree.net website.