Mapping Additional Domains with WordPress Multisite

domain-mapping.php

<?php

class DomainMapper {

  /**
   * Where we store the fully qualified domain URL throughout the lifecycle of the WordPress stack.
   */
  private $domain = null;

  /**
   * Where we store the active site object discovered during the network_short_circuit function.
   */
  private $site = null;

  public function __construct() {

    add_filter( 'pre_get_site_by_path', array( $this, 'network_short_circuit' ), 10, 2 );
    
    add_filter( "blog_option_siteurl", array( $this, 'hijack_blog_option' ), 99, 2 );
    add_filter( "blog_option_home", array( $this, 'hijack_blog_option' ), 99, 2 );

    add_filter( "pre_option_siteurl", array( $this, 'hijack_get_option' ), 99, 1 );
    add_filter( "pre_option_home", array( $this, 'hijack_get_option' ), 99, 1 );
    
    add_filter( 'get_canonical_url', array( $this, 'assign_canonical_url', 11 ) );

  }
  
  public function network_short_circuit( $site, $domain ) {
    
    // first let's utilize WordPress's object cache to see if this domain has already been found
    // why do a sql lookup if you don't need to?
    $cached = wp_cache_get( 'domain_site__' . sanitize_title( $domain ) );
    if ( $cached !== false ) {
      $this->setup_FQD( $domain );
      $this->site = $cached;
      return $cached;
    }

    // The following commented code is how it works with my own setup.
    // It will NOT work with yours, unless you're storing domains in the blogmeta table,
    // so either do it exactly the same as me, or replace this query with your own process.
    
    /* REPLACE:
    
    $site_id = $wpdb->get_var( $wpdb->prepare(
      "SELECT blog_id FROM {$wpdb->blogmeta} WHERE `meta_key` = 'domain' AND `meta_value` = %s",
      $domain
    ) );
    
    */

    if ( ! empty( $site_id ) ) {
      $site         = \WP_Site::get_instance( $site_id );
      $site->domain = $domain;
      
      $this->site = $site;      
      $this->setup_FQD( $domain );
    }
    
    // Store our discovery in the cache for quick lookup later.
    wp_cache_set( 'domain_site__' . $domain, $site );
    
    return $site;
    
  }
  
  /**
   * Stores the fully qualified domain address in our class for retrieval.
   */
  private function setup_FQD( $domain ) {
    
    $protocol = ( ! empty($_SERVER['HTTPS'] ) && $_SERVER['HTTPS'] !== 'off' || $_SERVER['SERVER_PORT'] == 443 ) ? "https://" : "http://";
    $this->domain = $protocol . $domain;
    
  }
  
  public function hijack_blog_option( $value, $id ) {

    if ( ! empty( $this->domain ) && absint( $id ) === $this->site->ID ) {
      return $this->domain;
    }

    return $value;

  }
  
  public function hijack_get_option( $value ) {

    if ( ! empty( $this->domain ) ) {
      return $this->domain;
    }

    return $value;

  }
  
  /**
   * From the get_canonical_url filter, assigns the correct domain to the canonical tag.
   * Will require edits based on your WordPress configuration.
   */
  public function assign_canonical_url( $url ) {

    // You'll need to figure out what the primary domain of each site is on your own, the
    // following only works if you're storing the primary domain in the wp_blogmeta table.
    
    /* REPLACE:
    
    $primary_domain = get_site_meta( get_current_blog_id(), 'domain', true ); 
    
    */
    if ( empty( $primary_domain ) ) {
      return $url;
    }

    // I always force the https, but YMMV
    return 'https://' . $primary_domain . $_SERVER['REQUEST_URI'];

  }
  
}

new DomainMapper();