A starter WP REST API controller for creating a Dynamic App for Help Scout. http://developer.helpscout.net/custom-apps/dynamic/

helpscout-rest-controller.php

<?php
/**
 * Help Scout REST controller class.
 *
 * @license GPL-2.0+
 * @link    http://developer.helpscout.net/custom-apps/dynamic/
 */
class HelpScout_REST_Controller {
	/**
	 * Secret key.
	 *
	 * @todo Enter the secret key for your custom app. If you need to create
	 *       one, use `wp_generate_password( 40 );`.
	 *
	 * @var string
	 */
	const SECRET_KEY = '{secret key goes here}';

	/**
	 * Register routes.
	 */
	public function register_routes() {
		register_rest_route( 'helpscout', '/v1', [
			[
				'methods'             => WP_REST_Server::CREATABLE,
				'callback'            => [ $this, 'handle_request' ],
				'permission_callback' => [ $this, 'check_permissions' ],
				'args'                => [
					'customer' => array(
						'required' => true,
					),
					'mailbox'  => array(
						'required' => true,
					),
					'ticket'   => array(
						'required' => true,
					),
				],
			],
		] );
	}

	/**
	 * Verify the signature.
	 *
	 * @since 1.0.0
	 * 
	 * @param  WP_REST_Request $request Request instance.
	 * @return boolean
	 */
	public function check_permissions( $request ) {
		global $wp_rest_server;
		$raw_data  = $wp_rest_server->get_raw_data();
		$signature = base64_encode( hash_hmac( 'sha1', $raw_data, self::SECRET_KEY, true ) );
		return hash_equals( $request->get_header( 'x-helpscout-signature' ), $signature );
	}

	/**
	 * Handle the request.
	 * 
	 * @param  WP_REST_Request $request Request instance.
	 * @return array|WP_REST_Response
	 */
	public function handle_request( $request ) {
		$user = $this->find_user( $request['customer'] );

		if ( is_wp_error( $user ) ) {
			return $this->error_to_response( $user );
		}

		$data = array(
			'html' => $this->get_html( $user ),
		);

		return $data;
	}

	/**
	 * Build the HTML for the custom app.
	 *
	 * @since 1.0.0
	 * 
	 * @param  WP_User $user User object.
	 * @return string
	 */
	protected function get_html( $user ) {
		return ''; // @todo Build the HTML.
	}

	/**
	 * Find a user by email address.
	 * 
	 * @param  array $customer Array of customer data.
	 * @return WP_User|WP_Error
	 */
	protected function find_user( $customer ) {
		$emails = array_unique( array_merge( array( $customer['email'] ), $customer['emails'] ) );

		foreach ( $emails as $email ) {
			$user = get_user_by( 'email', $email );

			if ( $user ) {
				return $user;
			}
		}

		return new WP_Error( 'unknown_user', 'A user with that email address was not found.' );
	}

	/**
	 * Convert a WP_Error object into a response.
	 * 
	 * @param  WP_Error $error Error object.
	 * @return WP_REST_Response
	 */
	protected function error_to_response( $error ) {
		$data = array( 'html' => $error->get_error_message() );
		return new WP_REST_Response( $data, 200 );
	}
}