One thing common between different OSes is that they provide an
abstraction over graphics hardware to make developers' lives easier.
On Unix we have X Window system. On Win32 platforms - GDI. On Mac OS X
- Quartz.
Since GTK+ is supposed to work on multiple platforms, it has a library
called GDK (GTK+ Drawing Kit) which isolates GTK+ from the details of
the windowing system.
| XWindows | XDrawPoint() |
GDI | SetPixel() |
Quartz | You don't wanna know |
GDK | gdk_draw_point() |
PHP-GTK | gdk::draw_point() |