How to create a custom JSON layout from MySQL in PHP

I have a MySQL query in PHP that pulls back two columns of results

Column One is Label Column Two is Value

I have read http://nitschinger.at/Handling-JSON-like-a-boss-in-PHP but im struggling to understand how I can do the following JSON in PHP from MySQL

[
  {
    key: "Cumulative Return",
    values: [
      { 
        "label": "One",
        "value" : 29.765957771107
      } , 
      { 
        "label": "Two",
        "value" : 0
      } , 
      { 
        "label": "Three",
        "value" : 32.807804682612
      } , 
      { 
        "label": "Four",
        "value" : 196.45946739256
      } , 
      { 
        "label": "Five",
        "value" : 0.19434030906893
      } , 
      { 
        "label": "Six",
        "value" : 98.079782601442
      } , 
      { 
        "label": "Seven",
        "value" : 13.925743130903
      } , 
      { 
        "label": "Eight",
        "value" : 5.1387322875705
      }
    ]
  }
]

I can manually write a loop to output the raw text to form a JSON like above but I really want to use json_encode

<?php
$hostname = 'localhost'; //MySQL database
$username = 'root'; //MySQL user
$password = ''; //MySQL Password

try {
    $dbh = new PDO("mysql:host=$hostname;dbname=etl", $username, $password);

    $query = "select label, value from table";

    $stmt = $dbh->prepare($query);
    $stmt->execute();

    $result = $stmt->fetchAll(PDO::FETCH_ASSOC);

    if (!empty($result)) {
        echo '[
        {
          key: "Cumulative Return",
          values: [';

        for ($row = 0; $row < count($result); $row++) {
            echo "{";
            echo '"label": "' . $result[$row]['Label'] . '",';
            echo '"Value": ' . $result[$row]['Value'];
            echo "},";
            echo '       ]
      }
    ]';
        }

    }
}
catch (PDOException $e) {
    echo $e->getMessage();
}

?> 

Can this be done? if so how?

Answer

Try this:

$hostname = 'localhost'; //MySQL database
$username = 'root'; //MySQL user
$password = ''; //MySQL Password

try {
    $dbh = new PDO("mysql:host=$hostname;dbname=etl", $username, $password);

    $query = "select label, value from table";

    $stmt = $dbh->prepare($query);
    $stmt->execute();

    $result = $stmt->fetchAll(PDO::FETCH_ASSOC);
    $values = array();

    for($row = 0; $row < count($result); $row++)
    {
        $values[] = array('label' => $result[$row]['Label'], 'value' => $result[$row]['Value']);
    }

    $to_encode = array(
                        array('key' => 'Cumulative Return', 
                              'values' => $values;
                              )
                        );
    echo json_encode($to_encode);

} catch (PDOException $e) {
    echo $e->getMessage();
}