How to receive JSON object in PHP?

I'm trying to send a JSON object from an AJAX request to my server.

I'm doing this with JQuery like this:

  $.ajax({
    type: "POST",
    url: settings.ajax.post,
    dataType: 'json',
    data: basket.aggregate(Basket.EXPORT_JSON, qty),
    success: function(data, textStatus, jqXHR) {
      if (typeof settings.ajax.success == "function") settings.ajax.success(data, textStatus, jqXHR);
    },
    error: function(jqXHR, text, e) {
      if (typeof settings.ajax.error == "function") settings.ajax.error(jqXHR, text, e);
    }
  });

The url is pointed to this file on the server:

<?php

$to = "<my-email-address>";
$subject = "JSON test";
$message = "POST dump:\n\n";

foreach($_POST as $key=>$value)
    {
        $message .= $key . ":" . $value;
    }

mail ($to, $subject, $message);

exit;
?>

But the POST var seems to be empty, even though in Firebug I can see that the correct data was sent to the server:

After each request is sent, the ajax error function is called, with an undefined error (I guess because there was no reply from the server? Or I don't know?)