How can I use JSONP to download client-side javascript objects?

Question

I'm trying to get client-side javascript objects saved as a file locally. I'm not sure if this is possible.

The basic architecture is this:

1. Ping an external API to get back a JSON object
2. Work client-side with that object, and eventually have a "download me" link
3. This link sends the data to my server, which processes it and sends it back with a mime type application/json, which (should) prompt the user to download the file locally.

Right now here are my pieces:

Server Side Code

<?php
$data = array('zero', 'one', 'two', 'testing the encoding');  
$json = json_encode($data);  
//$json = json_encode($_GET['']); //eventually I'll encode their data, but I'm testing
header("Content-type: application/json"); 
header('Content-Disposition: attachment; filename="backup.json"'); 
echo $_GET['callback'] . ' (' . $json . ');';
?>

Relevant Client Side Code

$("#download").click(function(){
    var json = JSON.stringify(collection); //serializes their object
    $.ajax({
       type: "GET",
       url: "http://www.myURL.com/api.php?callback=?", //this is the above script
       dataType: "jsonp",
       contentType: 'jsonp',
       data: json,
       success: function(data){
           console.log( "Data Received: " + data[3] );
       }
    });
    return false;
});

Right now when I visit the api.php site with Firefox, it prompts a download of download.json and that results in this text file, as expected:

 (["zero","one","two","testing the encoding"]);

And when I click #download to run the AJAX call, it logs in Firebug

Data Received: testing the encoding

which is almost what I'd expect. I'm receiving the JSON string and serializing it, which is great. I'm missing two things:

The Actual Questions

1. What do I need to do to get the same prompt-to-download behavior that I get when I visit the page in a browser
2. (much simpler) How do I access, server-side, the json object being sent to the server to serialize it? I don't know what index it is in the GET array (silly, I know, but I've tried almost everything)

Answer 1

1. You need to tell the browser to visit the page, usually by setting window.location.
2. Since it's a string, it will be sent as part of the raw query string. Try looking in $_SERVER['QUERY_STRING'] for it.