PHP MapScript zoomscale optional parameter

[Abe Gillespie]

According to the docs the oGeorefExt parameter of mapObj::zoomscale is
optional.  How do I send in an optional parameter in PHP?  I've tried
both of the following with no success:

$map->zoomscale($scale, $pnt, $w, $h, null);
and
$map->zoomscale($scale, $pnt, $w, $h);

Please help if you can.  Also, and slightly OT, you need to pass in
the height and width of the image.  This doesn't make sense to me. 
Shouldn't the current height and width of the mapObj be used???

Thanks for the help.
-Abe