PayPal API:异常:访问 https://api.sandbox.paypal.com/v1/oauth2/to

我收到的特定401错误消息是：

"error":"invalid_client","error_description":"The client credentials are invalid"}"

此错误未在PayPal文档中的任何位置列出。 我确定我正在使用测试凭据并使用正确的沙盒端点。 当我尝试获取访问令牌时发生错误。

这是检索访问令牌的类：

private function _generateAccessToken($config) {
$base64ClientID = base64_encode($this->clientId . ":" . $this->clientSecret);                           
$headers = array(
    "User-Agent" => PPUserAgent::getValue(RestHandler::$sdkName, RestHandler::$sdkVersion), 
    "Authorization" => "Basic " . $base64ClientID,
    "Accept" => "*/*"
);      
$httpConfiguration = $this->getOAuthHttpConfiguration($config); 
$httpConfiguration->setHeaders($headers); 
$connection = PPConnectionManager::getInstance()->getConnection($httpConfiguration, $config);
//print_r($connection); die;
$res = $connection->execute("grant_type=client_credentials");       
$jsonResponse = json_decode($res, true);
if($jsonResponse == NULL || 
        !isset($jsonResponse["access_token"]) || !isset($jsonResponse["expires_in"]) ) {
    $this->accessToken = NULL;
    $this->tokenExpiresIn = NULL;   
    $this->logger->warning("Could not generate new Access token. Invalid response from server: " . $jsonResponse);      
} else {
    $this->accessToken = $jsonResponse["access_token"];
    $this->tokenExpiresIn = $jsonResponse["expires_in"];
}
$this->tokenCreateTime = time();
return $this->accessToken;
}

这是我print_r时拥有的$connection变量（我删除了授权字符串）：

PayPal'Core'PPHttpConnection Object( [httpConfig:PayPal'Core'PPHttpConnection:private] => PayPal'Core'PPHttpConfig Object ( [headers:PayPal'Core'PPHttpConfig:private] => Array ( [User-    Agent] => PayPalSDK/rest-sdk-php 0.6.0 (lang=PHP;v=5.4.21;bit=64;os=Linux_2.6.18-    308.16.1.el5;machine=x86_64;openssl=0.9.8e-fips-rhel5;curl=7.24.0) [Authorization] => Basic     REMOVED AUTHORIZATION STRING == [Accept] => */* ) [curlOptions:PayPal'Core'PPHttpConfig:private]     => Array ( [32] => 3 [78] => 30 [19913] => 1 [13] => 60 [10018] => PayPal-PHP-SDK [10023] => Array ( ) [81] => 2 [64] => 1 ) [url:PayPal'Core'PPHttpConfig:private] => https://api.sandbox.paypal.com/v1/oauth2/token [method:PayPal'Core'PPHttpConfig:private] => POST [retryCount:PayPal'Core'PPHttpConfig:private] => 1 ) [logger:PayPal'Core'PPHttpConnection:private] => PayPal'Core'PPLoggingManager Object ( [loggerName:PayPal'Core'PPLoggingManager:private] => PayPal'Core'PPHttpConnection [isLoggingEnabled:PayPal'Core'PPLoggingManager:private] => 1 [loggingLevel:PayPal'Core'PPLoggingManager:private] => 3 [loggerFile:PayPal'Core'PPLoggingManager:private] => PayPal.log ))

据我所知，我有正确的凭据，正确的端点，不确定它还是什么？