1) I think this should be named Gtk2_VarDump. It will help keep things more organized. Packages in Gtk will be PHP-GTK 1 while packages in Gtk2 will be PHP-GTK 2. That way people using Gtk_VarDump with PHP-GTK 1 won't be as likely to upgrade to a package they can't use.
2) Lines 124 & 125 you add two GtkTreeViews to GtkScrolledWindows using add_with_viewport. GtkTreeView doesn't need a view port. See the second paragraph here: http://developer.gnome.org/doc/API/2.0/gtk/GtkScrolledWindow.html#gtk-scrolled-window-add-with-viewport
3) This is a minor point but can you add more inline comments.