jochem

with the caveat that it'll only work as a GET request (which probably means no cigar for you),
and requires that your php is setup to allow open urls (ini setting 'allow_url_fopen')

$response = file_get_contents('http://your.iis.machine/some/path/some_isapi.dll?foo=bar');

otherwise the fsockopen() route is all that seems to be open to you, given that curl reliance
is a no-no. - the biggest factor in the response speed is likely to be the server at the other
end (assuming the network connection between the 2 machines is generally ok), so if things
are slow you might consider caching the results locally (again, assuming that this is feasable.)

is the isapi extension on the same machine?
if so maybe it has a COM interface with which you can talk to it?

http://php.net/com

no idea if that is feasible - I've never really used IIS, let alone had to deal with isapi extensions.