var data = {
    object1: 15,
    object2: {
        element1: 'test',
        element2: 3,
        element3: undefined,
        element4: {
            element5: 11,
        }
    }
};

$data = array(
    'object1' => 15,
    'object2' => array(
        'element1' => 'test',
        'element2' => 3,
        'element4' => array(
            'element5' => 11,
        )
    ),
);

$js = <<<JSOBJ
{
    "object1": 15,
    "object2": {
        "element1": "test",
        "element2": 3,
        "element3": null,
        "element4": {
            "element5": 11
        }
    }
}
JSOBJ;

$php = json_decode($js, true);

echo var_export($php, true);

array (
  'object1' => 15,
  'object2' =>
  array (
    'element1' => 'test',
    'element2' => 3,
    'element3' => NULL,
    'element4' =>
    array (
      'element5' => 11,
    ),
  ),
)