Can extension A get access to a resource created by extension B? For example- if I wanted to add in a new image function, could I get access to the GD extension's gdImage struct?
When the extension exporting the resource is compiled in staticly this is trivial (notice what PDO does in this respect). Just include the header file and go.
#ifdef HAVE_GD
#include "ext/gd/php_gd.h"
/* blah blah blah */
#endif /* HAVE_GD */
When it's compiled as a shared ext you need to do some extra magic involving the extension loading order magic that.... may be ready? Not sure I havn't kept up with the status of this project.
I'm actually planning on putting support for exporting GD resources into my ogg/theora extension at some point (export frames from a video stream, or create a video stream from a sequence of frames). Though I expect you'll have yours in place before I get to that point.
-Sara