Sending JSON via AJAX to PHP using jQuery

I am trying to send JSON to a PHP file using jQuery AJAX, basically what I am trying to do is get the values and id's of a bunch of child elements and then assign them to a JSON object and then send that object via ajax to the PHP file which would then process it and enter it into a database.

Here is my code,

Javascript/jQuery:

function test(){
    var selects = $('#systems_wrapper').find('.dropDowns');
    var newArray = new Array();

    selects.each(function(){
        var id = $(this).attr('id');
        var val = $(this).val();
        var o = { 'id': id, 'value': val };

        newArray.push(o);
    });

    $.ajax({
            type: "POST",
            url: "qwer.php",
            dataType: 'json',
            data: { json: newArray }
        });

}

PHP:

<?php
    $json = $_POST['json'];
    $person = json_decode($json);

    $file = fopen('test.txt','w+');
    fwrite($file, $person);
    fclose($file);

    echo 'success?';
?>

It creates the file, but it is completely blank, any idea what it could be?

Thanx in advance!