PHP:Dropbox 令牌已过期或不存在

我正在尝试将文件从本地主机复制到Dropbox帐户，一旦获得令牌，我正在使用curl生成令牌，我正在使用它上传文件，但是我收到错误消息说令牌已过期或不存在。

这是我的代码：

## 步骤1.php ##

  <?php
    session_start();
    require_once('config.php');
    $step_2_url = rawurlencode("http://nuftp.com/sanjeev/step-2.php");
   $ch = curl_init(); 
     $headers = array( 'Authorization: OAuth oauth_version="1.0",                     
  oauth_signature_method="PLAINTEXT", oauth_consumer_key="' . $app_key . '",  
  oauth_signature="' . $app_secret . '&"' );
  curl_setopt( $ch, CURLOPT_HTTPHEADER, $headers ); 
   curl_setopt( $ch, CURLOPT_URL,  
  "https://api.dropbox.com/1/oauth/request_token" );  
    curl_setopt( $ch, CURLOPT_RETURNTRANSFER, TRUE );  
     $request_token_response = curl_exec( $ch );
   error_log(var_export($request_token_response,true));

   parse_str( $request_token_response, $parsed_request_token );
   $json_access = json_decode( $request_token_response );
  if ( isset( $json_access->error ) ) {
    echo '<br><br>FATAL ERROR: ' . $json_access->error . '<br><br>';
    die();
    }
   $_SESSION['myapp'] = array();
   $_SESSION['myapp']['oauth_request_token'] =       
    $parsed_request_token['oauth_token'];
   $_SESSION['myapp']['oauth_request_token_secret'] = 
  $parsed_request_token['oauth_token_secret'];
  header( 'Location: https://www.dropbox.com/1/oauth/authorize?oauth_token=' 
. $parsed_request_token['oauth_token'] . '&oauth_callback=' . $step_2_url );

## 步骤2.php ##

       <?php
       session_start();
      require_once('config.php');
     include 'dropbox-sdk-php-1.1.4/lib/Dropbox/autoload.php';
   use 'Dropbox as dbx;

  $step_3_url = "step-3.php";
     if ( isset( $_GET['oauth_token'] ) && isset( $_GET['uid'] ) && isset(   
     $_SESSION['myapp'] ) ) {
  $ch = curl_init(); 
  $headers = array( 'Authorization: OAuth oauth_version="1.0",  
    oauth_signature_method="PLAINTEXT", oauth_consumer_key="' . $app_key .  
   '", oauth_token="'  .$_GET['oauth_token'] . '", oauth_signature="' .   
  $app_secret . '&' . $_SESSION['myapp']['oauth_request_token_secret'] . '"'  
  );
   curl_setopt( $ch, CURLOPT_HTTPHEADER, $headers ); 
  curl_setopt( $ch, CURLOPT_URL, 
  "https://api.dropbox.com/1/oauth/access_token" );  
   curl_setopt( $ch, CURLOPT_RETURNTRANSFER, TRUE );  
    $access_token_response = curl_exec( $ch );
    error_log(var_export($request_token_response,true));
     parse_str( $access_token_response, $parsed_access_token );
    error_log( $access_token_response );
     $json_access = json_decode( $access_token_response );

   if ( isset( $json_access->error ) ) {
    echo '<br><br>FATAL ERROR: ' . $json_access->error . '<br><br>';
    die();
   }
   $_SESSION['myapp']['uid'] = $parsed_access_token['uid'];
   $_SESSION['myapp']['oauth_access_token'] =      
    $parsed_access_token['oauth_token'];
    $_SESSION['myapp']['oauth_access_token_secret'] = 
    $parsed_access_token['oauth_token_secret'];

   $dbxClient = new dbx'Client($_GET['oauth_token'], "PHP-Example/1.0");
   $accountInfo = $dbxClient->getAccountInfo();
     print_r($accountInfo);
      $f = fopen("working-draft.txt", "rb");
     $result = $dbxClient->uploadFile("/working-draft.txt",  
   dbx'WriteMode::add(), $f);
   fclose($f);
   print_r($result);
   $folderMetadata = $dbxClient->getMetadataWithChildren("/");
    print_r($folderMetadata);

    $f = fopen("working-draft.txt", "w+b");
    $fileMetadata = $dbxClient->getFile("/working-draft.txt", $f);
    fclose($f);
     print_r($fileMetadata);

    }

有人请建议我该怎么做，我哪里出错了。