set map to scale

[joerg pfeiffer]

hello,

At the moment, I am zooming my map.
But I would like to set my map to a specific scale with mapscript.
So button 1 sets to 100%,
Button 2 to 50%....
How do I do this?
$map->scale = xxx doesn't work.

Another question about this:
When my worldmap (-180,-90,180,90) is 100%, my map->scale is 147264446.33766
Why this large number? Where does it come from?

thanks
Joerg