PHP Codeigniter中的嵌套JSON对象循环

I'm having problems creating nested object looping from Controller . This is the data that i'm currently using

What i've already tried is :

function getCoordinate() {

 // Distict SQL Query for first loop 
     (SELECT DISTINCT(id_shipment) as id_shipment FROM tbl_tracking)

    $id_shipment = $this->Maps_model->getIdShipment(); 

 // First Loop
      foreach ($id_shipment as $value) {
          // The object i want to loop for views
          $json = array(
                    'type' => 'Feature',
                    'properties' => json_decode('{}'),
                    'geometry' => array(
                    'type' => 'LineString',
                    'coordinates' => array()
                  ));

 //Query for getting coordinate
   (SELECT a.latitude,a.latitude,b.ship_name from tbl_tracking a
    LEFT JOIN tbl_shipment b on a.id_shipment = b.id
    WHERE a.id_shipment = $value['id_shipment']);

      $coordinate = $this->Maps_model->getCoordinate($value['id_shipment']);

 //Second Loop inside The First Loop
        foreach ($coordinate as $key) {
          $json['geometry']['coordinates'] = array (  
                (float)$key['latitude'],
                (float)$key['longitude']
          );
        }
    echo json_encode($json);
    }
}

The code provided above is not generating JSON Object . How do i generate the right loop for generating JSON Object ?

You did not provide much information, but try to do something like this.

function getCoordinate()
{

    /* ... */
    $coordinates = [];

    // Second Loop inside The First Loop

    foreach($coordinate as $key) {
      /* ************* */
      // note the "[]" to add new items
      /* ************* */
      $coordinates[] = array(
        (float)$key['latitude'],
        (float)$key['longitude']
      );
    }
    $json['coordinates'] = $coordinates;
    echo json_encode($json);
  }
}