well, yer POST-ed data is alwasy passed in the global associative array $HTTP_POST_VARS (provided track_vars is on... but it should be) you can get them with
while (list($key, $value) = each($HTTP_POST_VARS))
{
echo "$key = $value<br>";
}
so, by deduction (induction?) you should be able to use putenv() to set up said array. viz:
putenv("HTTP_POST_VARS[\"FOO\"]=$bar");
you may need to set the request method, though, with
putenve("REQUEST_METHOD=POST");
of course this is pure conjecture on my part. if i'm talking through my hat, please advise me!