PayPal\Auth\Oauth\OAuthRequest::sign_request PHP Method

    public function sign_request($signature_method, $consumer, $token)
    {
        $empty = false;
        $msg = array();
        if ($token->key == null) {
            $msg[] = 'Token key';
        }
        if ($token->secret == null) {
            $msg[] = 'Token secret';
        }
        if ($consumer->key == null) {
            $msg[] = 'Consumer key';
        }
        if ($consumer->secret == null) {
            $msg[] = 'Consumer secret';
        }
        if ($this->http_url == null) {
            $msg[] = 'Endpoint';
        }
        if ($this->http_method == null) {
            $msg[] = 'HTTP method';
        }
        if (count($msg)) {
            throw new OAuthException('Enter valid ' . implode(',', $msg));
        }
        $this->set_parameter("oauth_signature_method", $signature_method->get_name(), false);
        $signature = $this->build_signature($signature_method, $consumer, $token);
        $this->set_parameter("oauth_signature", $signature, false);
    }