PHP and JQuery - PHP json_encoded 2D array doesn't arrive in JQuery Ajax

I am trying to create a demo to get data from a PHP script that runs a SQL query, takes the associated array and json_encodes's it and returns it to the JQuery ajax caller in the calling php file. For some reason it never arrives (check with Firebug). But if I manually create a 2D array, json_encode it, it works fine. I am completely stumped why my array never makes it from SQL but it does if I just hand type it. I have diffed the resulting strings and they are exactly the same.

Code:

...snip...
$.ajax({
        type: "GET",
        url: "getclients.php",
        data: { username: $('#staff_list').val() },
        //contentType: "application/json",
        dataType: 'json',
        success: function(results) {
            console.log("results");
        },
        fail: function() {
            console.log("fail!!");
        },
        error: function(r, e, m) {
            console.log("error");
            //console.dir(r);
            console.log(e + ', ' + m);
        }
    })
    .done(function(data) {
        console.log("done");
        //console.log(data);
    });
    console.log("done with change detection...");    
...snip...

PHP File:

<?php
header('Content-Type: application/json');
error_reporting(0); // prevents a notice from breaking ajax
//session_start();
$username = $_GET['username'];
$json = array();
$test = array(
    array("id"=>4,"first_name"=>"Miles","last_name"=>"O'Brian"),
    array("id"=>5,"first_name"=>"Jean Luc","last_name"=>"Picard"),
    array("id"=>6,"first_name"=>"Reginald","last_name"=>"Barclay")
          );

$mysqli = new mysqli('mydomain', 'myuser', 'mypassword', 'mydb');
if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    exit();
}
if ($stmt = $mysqli->prepare("SELECT `id`, `first_name`, `last_name` FROM client WHERE provider_username = ?")) {
    $stmt->bind_param('s', $username);
    $stmt->execute();
    $result = $stmt->get_result();


    while ($row = $result->fetch_assoc()) {
        $json[] = $row;
    }

    $result->close();
}

$mysqli->close();
echo json_encode($json);
echo json_encode($test);// this works
?>    

Any help with this would be appreciated. I am new to PHP but not to programming so I tried everything I can think of (including the manual and Google) and am just completely stumped.