如何使用格式将JSON写入txt文件？

我有JSON格式的内容，例如：

{"rand":"7542868","action":"get","identity":{"name":"Jack","password":"secret"},"key":"o8sh769f79"} 而且我知道怎么破译它。但是如何在txt文件中加入像下面这样优美的格式呢？

{
  "rand":"7542868",
  "action":"get",
  "identity":
  {
    "name":"Jack",
    "password":"secret"
  },
  "key":"o8sh769f79"
}

PHP Version 5.5.27

我创建的方法：

static function ApiLog($type ,$data)
    {
        $data = json_encode($data, JSON_PRETTY_PRINT);
        $myfile = fopen(__DIR__."/log/".date('Y-m-d').".txt", "w") or die("Unable to open file!");
        $entry = date("H:i:s")." ".$type." ".$data."
";
        fwrite($myfile, $entry);
        fclose($myfile);
    }

在$data中输入：

{"rand":"7542868","action":"get","identity":{"name":"Jack","password":"secret"},"key":"o8sh769f79"}

输出的文件依然是静止直列：

11:34:23 <- "{\"rand\":\"7542868\",\"action\":\"get\",\"identity\":{\"name\":\"Jack\",\"password\":\"secret\"},\"key\":\"o8sh769f79\"}"

$file = fopen("yourfile.txt","w");
fwrite($file, json_encode($data_array, JSON_PRETTY_PRINT));
fclose($file);

http://php.net/manual/de/function.json-encode.php