id, $args = array() ) {
		// Check for 0, false, null and other empty values.
		if ( empty( $config_id ) ) {
			return null;
		}

		$config_id = intval( $config_id );

		// Check if config is trashed.
		if ( 'trash' === get_post_status( $config_id ) ) {
			return null;
		}

		// Arguments.
		$args = wp_parse_args(
			$args,
			array(
				'gateway_id' => get_post_meta( $config_id, '_pronamic_gateway_id', true ),
				'mode'       => get_post_meta( $config_id, '_pronamic_gateway_mode', true ),
			)
		);

		// Get config.
		$gateway_id = $args['gateway_id'];
		$mode       = $args['mode'];

		$integration = pronamic_pay_plugin()->gateway_integrations->get_integration( $gateway_id );

		if ( null === $integration ) {
			return null;
		}

		$gateway = $integration->get_gateway( $config_id );

		return $gateway;
	}

	/**
	 * Complement payment.
	 *
	 * @param Payment $payment Payment.
	 * @return void
	 */
	private static function complement_payment( Payment $payment ) {
		// Entrance Code.
		if ( null === $payment->entrance_code ) {
			$payment->entrance_code = uniqid();
		}

		// Key.
		if ( null === $payment->key ) {
			$payment->key = uniqid( 'pay_' );
		}

		// User ID.
		if ( null === $payment->user_id && is_user_logged_in() ) {
			$payment->user_id = get_current_user_id();
		}

		$origin_id = $payment->get_origin_id();

		if ( null === $origin_id ) {
			// Queried object.
			$queried_object    = \get_queried_object();
			$queried_object_id = \get_queried_object_id();

			if ( null !== $queried_object && $queried_object_id > 0 ) {
				$origin_id = $queried_object_id;
			}

			// Referer.
			$referer = \wp_get_referer();

			if ( null === $origin_id && false !== $referer ) {
				$post_id = \url_to_postid( $referer );

				if ( $post_id > 0 ) {
					$origin_id = $post_id;
				}
			}

			// Set origin ID.
			$payment->set_origin_id( $origin_id );
		}

		// Google Analytics client ID.
		if ( null === $payment->analytics_client_id ) {
			$payment->analytics_client_id = GoogleAnalyticsEcommerce::get_cookie_client_id();
		}

		// Customer.
		$customer = $payment->get_customer();

		if ( null === $customer ) {
			$customer = new Customer();

			$payment->set_customer( $customer );
		}

		CustomerHelper::complement_customer( $customer );

		// Email.
		if ( null === $payment->get_email() ) {
			$payment->email = $customer->get_email();
		}

		// Billing address.
		$billing_address = $payment->get_billing_address();

		if ( null !== $billing_address ) {
			AddressHelper::complement_address( $billing_address );
		}

		// Shipping address.
		$shipping_address = $payment->get_shipping_address();

		if ( null !== $shipping_address ) {
			AddressHelper::complement_address( $shipping_address );
		}

		// Version.
		if ( null === $payment->get_version() ) {
			$payment->set_version( pronamic_pay_plugin()->get_version() );
		}

		// Mode.
		$config_id = $payment->get_config_id();

		if ( null === $payment->get_mode() && null !== $config_id ) {
			$mode = get_post_meta( $config_id, '_pronamic_gateway_mode', true );

			$payment->set_mode( $mode );
		}

		// Issuer.
		if ( null === $payment->issuer ) {
			// Credit card.
			if ( PaymentMethods::CREDIT_CARD === $payment->method && filter_has_var( INPUT_POST, 'pronamic_credit_card_issuer_id' ) ) {
				$payment->issuer = filter_input( INPUT_POST, 'pronamic_credit_card_issuer_id', FILTER_SANITIZE_STRING );
			}

			// iDEAL.
			$ideal_methods = array( PaymentMethods::IDEAL, PaymentMethods::DIRECT_DEBIT_IDEAL );

			if ( \in_array( $payment->method, $ideal_methods, true ) && filter_has_var( INPUT_POST, 'pronamic_ideal_issuer_id' ) ) {
				$payment->issuer = filter_input( INPUT_POST, 'pronamic_ideal_issuer_id', FILTER_SANITIZE_STRING );
			}
		}

		/**
		 * If an issuer has been specified and the payment
		 * method is unknown, we set the payment method to
		 * iDEAL. This may not be correct in all cases,
		 * but for now Pronamic Pay works this way.
		 *
		 * @link https://github.com/wp-pay-extensions/gravityforms/blob/2.4.0/src/Processor.php#L251-L256
		 * @link https://github.com/wp-pay-extensions/contact-form-7/blob/1.0.0/src/Pronamic.php#L181-L187
		 * @link https://github.com/wp-pay-extensions/formidable-forms/blob/2.1.0/src/Extension.php#L318-L329
		 * @link https://github.com/wp-pay-extensions/ninjaforms/blob/1.2.0/src/PaymentGateway.php#L80-L83
		 * @link https://github.com/wp-pay/core/blob/2.4.0/src/Forms/FormProcessor.php#L131-L134
		 */
		if ( null !== $payment->issuer && null === $payment->method ) {
			$payment->method = PaymentMethods::IDEAL;
		}

		// Consumer bank details.
		$consumer_bank_details = $payment->get_consumer_bank_details();

		if ( null === $consumer_bank_details ) {
			$consumer_bank_details = new BankAccountDetails();
		}

		if ( null === $consumer_bank_details->get_name() && filter_has_var( INPUT_POST, 'pronamic_pay_consumer_bank_details_name' ) ) {
			$consumer_bank_details->set_name( filter_input( INPUT_POST, 'pronamic_pay_consumer_bank_details_name', FILTER_SANITIZE_STRING ) );
		}

		if ( null === $consumer_bank_details->get_iban() && filter_has_var( INPUT_POST, 'pronamic_pay_consumer_bank_details_iban' ) ) {
			$consumer_bank_details->set_iban( filter_input( INPUT_POST, 'pronamic_pay_consumer_bank_details_iban', FILTER_SANITIZE_STRING ) );
		}

		$payment->set_consumer_bank_details( $consumer_bank_details );

		// Payment lines payment.
		$lines = $payment->get_lines();

		if ( null !== $lines ) {
			foreach ( $lines as $line ) {
				$line->set_payment( $payment );
			}
		}
	}

	/**
	 * Start payment.
	 *
	 * @param Payment $payment The payment to start at the specified gateway.
	 * @param Gateway $gateway The gateway to start the payment at.
	 *
	 * @return Payment
	 *
	 * @throws \Exception Throws exception if gateway payment start fails.
	 */
	public static function start_payment( Payment $payment, $gateway = null ) {
		global $pronamic_ideal;

		// Complement payment.
		self::complement_payment( $payment );

		/**
		 * Filters the payment gateway configuration ID.
		 *
		 * @param int     $configuration_id Gateway configuration ID.
		 * @param Payment $payment          The payment resource data.
		 */
		$config_id = \apply_filters( 'pronamic_payment_gateway_configuration_id', $payment->get_config_id(), $payment );

		$payment->set_config_id( $config_id );

		// Create payment.
		$pronamic_ideal->payments_data_store->create( $payment );

		// Prevent payment start at gateway if amount is empty.
		$amount = $payment->get_total_amount()->get_value();

		if ( empty( $amount ) ) {
			$payment->set_status( PaymentStatus::SUCCESS );

			$payment->save();

			/**
			 * Return or throw exception?
			 *
			 * @link https://github.com/wp-pay/core/commit/aa6422f0963d9718edd11ac41edbadfd6cd07d49
			 * @todo Throw exception?
			 */

			return $payment;
		}

		// Gateway.
		$gateway = self::get_gateway( $payment->get_config_id() );

		if ( null === $gateway ) {
			$payment->set_status( PaymentStatus::FAILURE );

			$payment->save();

			return $payment;
		}

		// Recurring.
		if ( true === $payment->get_recurring() && ! $gateway->supports( 'recurring' ) ) {
			throw new \Exception( 'Gateway does not support recurring payments.' );
		}

		// Start payment at the gateway.
		try {
			$gateway->start( $payment );

			// Add gateway errors as payment notes.
			$error = $gateway->get_error();

			if ( $error instanceof \WP_Error ) {
				$message = $error->get_error_message();
				$code    = $error->get_error_code();

				if ( ! \is_int( $code ) ) {
					$message = sprintf( '%s: %s', $code, $message );
					$code    = 0;
				}

				throw new \Exception( $message, $code );
			}
		} catch ( \Exception $error ) {
			$message = $error->getMessage();

			// Maybe include error code in message.
			$code = $error->getCode();

			if ( $code > 0 ) {
				$message = \sprintf( '%s: %s', $code, $message );
			}

			// Add note.
			$payment->add_note( $message );

			// Set payment status.
			$payment->set_status( PaymentStatus::FAILURE );
		}

		// Save payment.
		$payment->save();

		// Schedule payment status check.
		if ( $gateway->supports( 'payment_status_request' ) ) {
			StatusChecker::schedule_event( $payment );
		}

		// Throw/rethrow exception.
		if ( $error instanceof \Exception ) {
			throw $error;
		}

		return $payment;
	}

	/**
	 * Get pages.
	 *
	 * @return array
	 */
	public function get_pages() {
		$return = array();

		$pages = array(
			'completed' => __( 'Completed', 'pronamic_ideal' ),
			'cancel'    => __( 'Canceled', 'pronamic_ideal' ),
			'expired'   => __( 'Expired', 'pronamic_ideal' ),
			'error'     => __( 'Error', 'pronamic_ideal' ),
			'unknown'   => __( 'Unknown', 'pronamic_ideal' ),
		);

		foreach ( $pages as $key => $label ) {
			$id = sprintf( 'pronamic_pay_%s_page_id', $key );

			$return[ $id ] = $label;
		}

		return $return;
	}

	/**
	 * Payment redirect URL.
	 *
	 * @param string  $url     Redirect URL.
	 * @param Payment $payment Payment.
	 * @return string
	 */
	public function payment_redirect_url( $url, Payment $payment ) {
		$url = \apply_filters( 'pronamic_payment_redirect_url_' . $payment->get_source(), $url, $payment );

		return $url;
	}

	/**
	 * Is debug mode.
	 *
	 * @link https://github.com/easydigitaldownloads/easy-digital-downloads/blob/2.9.26/includes/misc-functions.php#L26-L38
	 * @return bool True if debug mode is enabled, false otherwise.
	 */
	public function is_debug_mode() {
		$value = \get_option( 'pronamic_pay_debug_mode', false );

		if ( PRONAMIC_PAY_DEBUG ) {
			$value = true;
		}

		return (bool) $value;
	}
}