我正在尝试使用FCM向设备发送通知。在服务器端，我通过python中的FCM Admin SDK（$auth_token）和设备$token获得了授权令牌。在运行代码时，我得到以下函数：

function sendNotification($token, $title,$body){

    $command = escapeshellcmd('/usr/bin/python3 api/fcm.py');
    $auth_token = shell_exec($command); 

    // set request data
    $request_data = array(
    'message' => array(
        'token' => $token,
        'notification' => array(
            'body' => $body,
            'title' => $title
            )
        )
    );

    $fields = json_encode($request_data);
    $request_length = strlen($fields);
    print($request_length);

    // set headers
    $headers = array(
        'Content-Type: application/json',
        'Authorization: Bearer ' . $auth_token,
        'Content-Length: '. $request_length
    );

    $url = 'https://fcm.googleapis.com/v1/projects/black-queen/8eff3/messages:send';

    $ch = curl_init ();
    curl_setopt ( $ch, CURLOPT_URL, $url );
    curl_setopt ( $ch, CURLOPT_POST, true );
    curl_setopt ( $ch, CURLOPT_HTTPHEADER, $headers );
    curl_setopt ( $ch, CURLOPT_RETURNTRANSFER, true );
    curl_setopt ( $ch, CURLOPT_POSTFIELDS, $fields );
    curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);

    $result = curl_exec ( $ch );
    echo $result;

    // check for errors
    if(curl_errno($ch)) {
        print('Error: ' . curl_error($ch));
    }

    // close cURL session
    curl_close($ch);

    // print response
    print($response);
}

字符串
但是，我收到以下错误消息：这是一个错误。POST请求需要Content-length报头。我们只知道这些。”