rest-api-subdomain.php

<?php

/**
 * Plugin Name: WP REST API Subdomain
 * Plugin URI: https://wordpress.org/plugins/wp-rest-api-subdomain/
 * Author: Mark McWilliams
 * Author URI: https://profiles.wordpress.org/markmcwilliams/
 * License: GPLv2 or later
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 * Description: Move your RESTful API to a subdomain in WordPress
 * Version: 0.1.0
 */

class WP_REST_API_Subdomain {
	
	/**
	 * URL to use for subdomain
	 * 
	 * @since 0.1.0
	 */
	 private $subdomain_url = 'api.example.com';
	
	/**
	 * The main constructor
	 * 
	 * @since 0.1.0
	 */
	public function __construct() {
		
		// Add REST API URL filter
		add_filter( 'rest_url', 'add_the_subdomain' );
		
		// Add the new Rewrite Rule
		add_action( 'init', 'add_new_rewrite_rule' );
		
	}
	
	/**
	 * Add callback for filter
	 * 
	 * @since 0.1.0
	 */
	public function add_the_subdomain( $args ) {
		
		$args = array(
			$url = $subdomain_url;
		);
		
		return $args;
		
	}
	
	/**
	 * Add Rewrite Rule
	 * 
	 * @since 0.1.0
	 */
	public function add_new_rewrite_rule() {
		
		// Bail if not the correct URL
		if ( $_SERVER['HOST_NAME'] !== $subdomain_url ) {
			return;
		}
		
		add_rewrite_rule( '^(.*)$', 'index.php?rest_route=$1', 'top' );
		
	}
	
}

new WP_REST_API_Subdomain();

?>