write json to a file using jQuery and Ajax and PHP

I'm trying to write a json file based upon a filled in Form

So I used an external Library for jquery that converts the Form into legitimate JSON. When I console.log that output I do get a valid json data returned.

So whenever I pass the data into a php using $.ajax and write the content to a file the PHP saves the file but the inside it just says "NULL"

here is my AJAX:

$(document).ready(function() {
    var json = $("#user-form").serializeJSON();
    $.ajax({
        url: "writejson.php",
        type: "POST",
        data: json,
        processData: false,
        contentType: 'application/json'
    });
})

And here is my PHP:

<?php
    $myFile = "kiosk.json";
    $fh = fopen($myFile, 'w') or die("can't open file");
    fwrite($fh,var_export($_POST['data'], true));
    fclose($fh);
?>

and here is what the outputted file says:

NULL

I tried looking it up here first and tried numerous options but none of them seem to save the correct data. really strange.

Thanks in advance!