how to enable cross domain POST-ing in PHP?

As others have said, there should be no problem doing this. I would suggest replacing your test.php script with something simple, like this

<?php
echo "<pre>";
var_dump($_POST);
echo "</pre>";

You should find it works, which means the blame lies somewhere in the "real" script...

What you have should work fine - forms came before the same-origin policy - you're allowed to submit to different domains.

If I were to hazard a guess, I'd say there's a 301 or 302 redirect getting in there somehow? Follow the HTTP headers with Firebug for example to be sure.