Braintree\Http::useClientCredentials PHP Method

    public function useClientCredentials()
    {
        $this->_useClientCredentials = true;
    }

 public static function createGrant($gateway, $params)
 {
     $http = new Braintree\Http($gateway->config);
     $http->useClientCredentials();
     $response = $http->post('/oauth_testing/grants', array('grant' => $params));
     return $response['grant']['code'];
 }