Regardless of the GtkShadowType detailed here, the effect is similar to the box drawn using gtk::draw_box() and GTK_SHADOW_NONE.

The GdkRectangle used should be in the same position and have the same dimensions as those passed in the function, as otherwise only the overlap between the two rectangles will be displayed, or nothing at all if there is no overlapping area. The detail parameter actually does nothing here, but is needed for back compatibility. Passing it as null, or as any string you feel, is okay.

$rectangle = &new GdkRectangle($x, $y, $width, $height);
gtk::paint_flat_box($style, $gdkwindow, GTK_STATE_ACTIVE, GTK_SHADOW_NONE, 
$rectangle, $drawingarea, null, $x, $y, $width, $height);