[cairo] How to represent a hollow path? cairo_wrapper on php

[Fan Zhang]

Dear All,

I'm using cairo with php to draw PNG, I've tried with layers and masks 
to represent hollow path however the PNG generated differently.

For example, if to draw a shape of charactor 'O', I draw the outside 
path and fill it black, then the inside path. The inside path should be 
transparent since the surface color is also transparent.
If to fill the inside layer with alpha 0, the path just won't show. How 
should I do it?

-- 
Many thanks

Fan.