PayPal\Auth\OAuthTokenCredential::generateAccessToken PHP Method

    private function generateAccessToken($config, $refreshToken = null)
    {
        $params = array('grant_type' => 'client_credentials');
        if ($refreshToken != null) {
            // If the refresh token is provided, it would get access token using refresh token
            // Used for Future Payments
            $params['grant_type'] = 'refresh_token';
            $params['refresh_token'] = $refreshToken;
        }
        $payload = http_build_query($params);
        $response = $this->getToken($config, $this->clientId, $this->clientSecret, $payload);
        if ($response == null || !isset($response["access_token"]) || !isset($response["expires_in"])) {
            $this->accessToken = null;
            $this->tokenExpiresIn = null;
            PayPalLoggingManager::getInstance(__CLASS__)->warning("Could not generate new Access token. Invalid response from server: ");
            throw new PayPalConnectionException(null, "Could not generate new Access token. Invalid response from server: ");
        } else {
            $this->accessToken = $response["access_token"];
            $this->tokenExpiresIn = $response["expires_in"];
        }
        $this->tokenCreateTime = time();
        return $this->accessToken;
    }