public static function quiz_end( $request, $attempt = null ) {

		$err = new WP_Error();

		if ( ! $attempt ) {

			$student = llms_get_student();
			if ( ! $student ) {
				$err->add( 400, __( 'You must be logged in to take quizzes.', 'lifterlms' ) );
				return $err;
			}

			if ( ! isset( $request['attempt_key'] ) ) {
				$err->add( 400, __( 'Missing required parameters. Could not proceed.', 'lifterlms' ) );
				return $err;
			}

			$attempt = $student->quizzes()->get_attempt_by_key( sanitize_text_field( $request['attempt_key'] ) );

		}

		// record the attempt's completion.
		$attempt->end();

		// setup a redirect.
		$url = add_query_arg(
			array(
				'attempt_key' => $attempt->get_key(),
			),
			get_permalink( $attempt->get( 'quiz_id' ) )
		);

		return array(
			/**
			 * Filter the quiz redirect URL on completion
			 *
			 * Return an associative array containing at least the `$id` to cease execution and have
			 * the custom item returned via the `process_trash()` method.
			 *
			 * A successful deletion return should be: `array( 'id' => $id )`.
			 *
			 * A failure should contain an error message in a second array member:
			 * `array( 'id' => $id, 'error' => esc_html__( 'My error message', 'my-domain' ) )`.
			 *
			 * @since Unknown
			 *
			 * @param string            $url     The quiz redirect URL on completion.
			 * @param LLMS_Quiz_Attempt $attempt The quiz attempt.
			 */
			'redirect' => apply_filters( 'llms_quiz_complete_redirect', $url, $attempt ),
		);

	}