POST requests with WIRE

I've got a broad, simple question about sending POST requests with WIRE. (I'm pretty new to REST APIs: I know how to send HTML forms to PHP, I've played with John's excellent Parse example, and I really get GETs. However, POST requests are a little tricky.)

Here's what I've got:

I made a super simple php script that lives at http://tylr.us/testing/posttest.php. Here's the code:

<?php
<br/>
header('Content-Type: application/json');
<br/>
$data = array("key" => $_POST["value"]);
<br/>
echo json_encode($data);
<br/>
?>

This script does three things: gives itself a content-type of JSON, creates an array and adds a value that is POSTed as "value" to it, and encodes the array as JSON. I've been testing it with the Postman plug-in for Chrome and everything looks beautiful:



Now I'm trying to do two things from WIRE: pass the value through POST, and display the output. Super simple (I hope). Here's what I'm trying to do in WIRE:

<wire><datasources><datasource name="posttest" source="" query="//" httpmethod="post" providertype="json" /></datasources><actions><action name="posttest" oninit="yes"><assign property="datasource:posttest.postcontent" value="value=testing123" /><assign property="datasource:posttest.source" value="http://tylr.us/testing/posttest.php" /></action><action datasource="posttest" datasourceevent="querycomplete"><alert message="[datasource:posttest.1.value]" /></action></actions></wire>

When I try this, it doesn't work. Now, there are a couple of failure points: I could have my WIRE straight-up wrong. I'm assuming that WIRE can display the output of PHP pretending to be JSON, but that assumption may be wrong. But the extremely likely situation is that I don't completely understand datasources and/or POST.

If I had to guess, I would think I might need a header, but I have no idea where to start (beyond the aforementioned Parse example from John, but I don't know where Parse-specific things stop and POST/HTTP standards begin). Any tips? :)