我尝试在Android上用Flutter构建一个应用程序。readData()方法发送了'Authorization': 'Bearer $token' HTTP标头。问题是$token未正确发送，服务器上接收到Null。我尝试发送$token，${token}，纯文本令牌（字符串）-没有工作。
当我尝试从https://reqbin.com/post-online发送包含token作为纯文本的get请求时，它工作得很好。
你能告诉我是什么原因导致了这个问题吗？

Future readData() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    String token = prefs.getString('token').toString();

    print('Token : $token');     // prints $token OK

    final response = await get(
        Uri.parse('https://www.somehost.com/api.php'),
        headers: {
        'Content-Type': 'application/json',
        'Accept': 'application/json',
        'Authorization': 'Bearer $token',     // here token is null (read from server log)
        }
      );

    print(response.statusCode);

    var data = jsonDecode(response.body.toString());
    print(data);
    print(data['success']);
    print(data['user']['email']);
  }

API代码：

<?php
header("Access-Control-Allow-Origin: *");
header("Access-Control-Allow-Headers: access");
header("Access-Control-Allow-Methods: GET");
header("Content-Type: application/json; charset=UTF-8");
header("Access-Control-Allow-Headers: Content-Type, Access-Control-Allow-Headers, Authorization, X-Requested-With");

require __DIR__.'/classes/db.php';
require __DIR__.'/auth.php';

$allHeaders = getallheaders();
$db_connection = new Database();
$conn = $db_connection->dbConnection();
$auth = new Auth($conn, $allHeaders);

echo json_encode($auth->isValid());
?>