MySQL to JSON, how to get the right format?

So, I'm using a PHP-script to output MySQL to JSON, but I'm having trouble figuring out how to output the right JSON-format.

Here's the PHP-script:

$sql_query = "SELECT * FROM DiseaseData"; 
$res_sql = mysql_query($sql_query) or die(mysql_error());
$arr = array();


if(mysql_num_rows($res_sql) > 0){   

    ini_set('memory_limit', '-1');
    while($row_sql = mysql_fetch_assoc($res_sql)){

        $arr[] =  $row_sql;

        }
        $json = json_encode($arr);
        $file = '../../files/json/DiseaseData.json';
        file_put_contents($file, $json);

    }  
    ini_set('memory_limit', '-1');

Here's the outputted JSON-format:

[{
    "ID": "1",
    "Magnitude": "0.842",
    "County": "Alameda",
    "Disease": "E. coli O157",
    "lat": "37.7652",
    "lng": "-122.242"
}, {
    "ID": "2",
    "Magnitude": "1.520",
    "County": "Alameda",
    "Disease": "HIV",
    "lat": "37.7652",
    "lng": "-122.242"
}]

This is the JSON-format I'd like to have it in:

{
    "columns":[{
            "fieldName" : "ID",
            "position" : 1
    },
    {
            "fieldName" : "Magnitude",
            "position" : 2
    },
    {
            "fieldName" : "County",
            "position" : 3
    },
    {
            "fieldName" : "Disease",
            "position" : 4
    },
    {
            "fieldName" : "lat",
            "position" : 5
    },
    {
            "fieldName" : "lng",
            "position" : 6
    },]
    "data": [ 
        [ 1, 0.842, "Alameda", "E. coli O157", 37.7652, -122.242],
        [ 2, 1.520, "Alameda", "HIV", 37.7652, -122.242]
    ]
}