Highcharts drilldown json from php mysql

Question

Why cant I make my Highcharts drill down work when I change json data from hardcoded javscript below to parse from a php mysql ?

Codes are as below

json source hardcoded in java script that works

drilldowns = {
              name: 'Animals',
              data: [
                     ['Cows', 2],
                     ['Sheep', 3]
                    ]
             },

When I call json from the following php code it doesn't

<?php

$ret = array();
$mtbf = array();
$mtbf['name'] = "Animals";

$mtbf['data'] = array(
    array( 'Cows', 2),
    array( 'Sheep',3) );

array_push($ret,$mtbf);
echo json_encode($ret);

?>

I combined both source of json for easy trouble shooting and below works

$.get("/drilldown.php?name=" +e.point.name,  function(data) {

    drilldowns = {
            name: 'Animals',
            data: [
                ['Cows', 2],
                ['Sheep', 3]
            ]
    },

    series = drilldowns;

  alert(series);

    chart.addSeriesAsDrilldown(e.point, series);

});

But if I change to the series from drilldowns to data it doesn't work

series = data; 

Any help will be much appreciated

As per comments console log for data as follows

[{"name":"Animals","data":[["Cows",2],["Sheep",3]]}]

console log for drilldowns

Object {name: "Animals", data: Array(2)}

Answer 1

I managed to solve the problem by adding the header as well minor tweak on the array assignment and the drilldown worked.

<?php
  header("Content-Type: application/json");

  $mtbf = array();
  $mtbf['name'] = "Animals";
  $mtbf['data'] = array(array( 'Cows', 2),array( 'Sheep',3));

  //var_dump($mtbf);

  //array_push($ret,$mtbf);

  echo json_encode($mtbf);

  ?>