Buffer Query

[Abe Gillespie]

Is there a way in PHP, or a set of steps someone could describe, to do
a buffer query from a shape as opposed to just a point.  I'm looking
for something that would be like this:

MapObj::queryByShape(shapeObj shape, double buffer)

I wouldn't mind doing this by hand if there is no built in
functionality.  I imagine you have to loop through each line of the
polygon and offset them by the buffer distance away from the centroid.
Once that's done you'd have to extend all the offset lines so they
connect.  The theory's easy, but I'm lacking in the equations to
accomplish this.  Can anyone point me some place that will get me
started?

Thanks.
-Abe