I found the time to fix up my ffi extension so that it largely usable; the main features that are missing are support for arrays and closures (aka callbacks), but it should otherwise be good enough for some hacking.
ffi is multiplatform, and should work on a wide variety of unices as well as the known-good platforms: Linux/x86, Win32 and OSX.
If you want to try it on win32, you can download the PECL binary for windows; this is updated each time the PHP 5 snapshot is rebuilt--every 3 hours or so.
If you want to try it on other platforms, use
pear install ffi to install and build the code from source; you need to have previously installed PHP 5 to use ffi.