void setButton(Php::Parameters &params) {
//      Php::Value callback = params[0];
        _button = gtk_button_new_with_label(_buttonTitle);
        gtk_container_add(GTK_CONTAINER(_window), _button);
        g_signal_connect(G_OBJECT(_button), "clicked", G_CALLBACK(callback), NULL);
   }

   void callback(GtkButton *button, gpointer data) {
        g_print("testing");
   }

In file included from /usr/include/glib-2.0/gobject/gobject.h:28:0,
                 from /usr/include/glib-2.0/gobject/gbinding.h:29,
                 from /usr/include/glib-2.0/glib-object.h:23,
                 from /usr/include/glib-2.0/gio/gioenums.h:28,
                 from /usr/include/glib-2.0/gio/giotypes.h:28,
                 from /usr/include/glib-2.0/gio/gio.h:26,
                 from /usr/include/gtk-2.0/gdk/gdkapplaunchcontext.h:30,
                 from /usr/include/gtk-2.0/gdk/gdk.h:32,
                 from /usr/include/gtk-2.0/gtk/gtk.h:32,
                 from main.cpp:3:
main.cpp: In member function ‘void Gtk::setButton(Php::Parameters&)’:
/usr/include/glib-2.0/gobject/gclosure.h:70:41: error: invalid use of member function (did you forget the ‘()’ ?)
 #define G_CALLBACK(f)    ((GCallback) (f))
                                         ^
/usr/include/glib-2.0/gobject/gsignal.h:475:60: note: in definition of macro ‘g_signal_connect’
     g_signal_connect_data ((instance), (detailed_signal), (c_handler), (data), NULL, (GConnectFlags) 0)
                                                            ^
main.cpp:46:49: note: in expansion of macro ‘G_CALLBACK’
  g_signal_connect(G_OBJECT(_button), "clicked", G_CALLBACK(callback), NULL);
                                                 ^