Не срабатывает json_decode из массива

$json = '[{"id_event":1,"id_user":1,"id_pr":1}, {"id_event":2,"id_user":2,"id_pr":2}, {"id_event":3,"id_user":3,"id_pr":3}]';
$json = json_decode($json, true);
echo $json[1]['id_event'];

Результат:

2

Если у Вас json в массиве, то

$arr_val = array('{"id_event":1,"id_user":1,"id_pr":1}', '{"id_event":2,"id_user":2,"id_pr":2}', '{"id_event":3,"id_user":3,"id_pr":3}');
foreach($arr_val as $v)
{
  $json = json_decode($v, true);
  var_dump($json);
  echo '<br>';
  echo $json['id_event'];
  echo '<br>';
}

Результат:

array(3) { ["id_event"]=> int(1) ["id_user"]=> int(1) ["id_pr"]=> int(1) } 
1
array(3) { ["id_event"]=> int(2) ["id_user"]=> int(2) ["id_pr"]=> int(2) } 
2
array(3) { ["id_event"]=> int(3) ["id_user"]=> int(3) ["id_pr"]=> int(3) } 
3