WordPress的REST API发出解析请求数据的问题

Question

我有一个自定义插件设置来处理JSON请求

[
  {
    "id": "QWNjb3VudDpkZWFuQHRlY2hzdGFja3NvbHV0aW9ucy5jb20=",
    "firstName": "admin",
    "lastName": null,
    "categories": [
      "Q2F0ZWdvcnk6MmYyNTNjZDAtMTQ4Yi0xMWU5LTkyYTUtMWIwYjU5ZmExMDJl",
      "Q2F0ZWdvcnk6MmUxMjk0NTAtMTQ4Yi0xMWU5LTkyYTUtMWIwYjU5ZmExMDJl"
    ]
  },
  {
    "id": "QWNjb3VudDp0ZXN0ZW1haWxAdGVzdHNpdGUuY29t",
    "firstName": "test",
    "lastName": "user",
    "categories": [
      "Q2F0ZWdvcnk6MmUxMjk0NTAtMTQ4Yi0xMWU5LTkyYTUtMWIwYjU5ZmExMDJl"
    ]
  }
]

这是我的插件端点设置

add_action( 'rest_api_init', function () {
    register_rest_route( 't2mchat/v2', 'post_categories', array(
        'methods' => 'POST',
        'callback' => 'post_user_data'
    ));
});

这是处理函数

    global $wpdb;
    $parameters = $request->get_body();
    print_r($parameters[0]);exit;

但是，它继续输出以下消息：There was an error parsing JSON data. Unexpected end of JSON input。

将此行print_r($parameters[0]);exit; 更改为print_r(count($parameters));exit;会打印0，尽管我在数组中有2个元素。有想法吗？