array(2) {
  [0]=>
  object(PackingListData)#6 (8) {
    ["referencia"]=>
    string(10) "27713/5294"
    ["estilo"]=>
    string(5) "14376"
    ["bultos"]=>
    string(2) "26"
    ["total_bultos"]=>
    string(2) "28"
    ["piezas_teoricas"]=>
    string(2) "27"
    ["total_piezas"]=>
    string(2) "28"
    ["pack_id"]=>
    string(1) "5"
    ["id"]=>
    string(2) "10"
  }
  [1]=>
  object(PackingListData)#7 (8) {
    ["referencia"]=>
    string(10) "27713/5294"
    ["estilo"]=>
    string(5) "14377"
    ["bultos"]=>
    string(1) "2"
    ["total_bultos"]=>
    string(2) "28"
    ["piezas_teoricas"]=>
    string(1) "3"
    ["total_piezas"]=>
    string(1) "4"
    ["pack_id"]=>
    string(1) "5"
    ["id"]=>
    string(2) "11"
  }
}

<?php

header('Content-type:application/json;charset=utf-8');

$id = 5;

$packing = PackingListData::getPackingList($id);

$arr = array();

foreach ($packing as $pack) {

$arr = array( 'id'=>$pack->id,
                'referencia' =>$pack->referencia,
                'estilo'=>$pack->estilo,
                'bultos'=>$pack->bultos,

              );
    echo  json_encode($arr);
}

{"id":"10","referencia":"27713/5294","estilo":"14376","bultos":"26"}{"id":"11","referencia":"27713/5294","estilo":"14377","bultos":"2"}