Mapping Domains to Permalinks (not multisite)

Question

If you set domain.com as an alias of original.com, in WordPress you have to do nothing to make it works.

The problem is the countrary: once in DNS the 2 domains are aliases, every url of your WordPress will be accessible via user defined domains: domain.com/any/wp/url, but also domain2.com/any/wp/url, domain3.com/any/wp/url and so on…

So, what you have to do, is

Check if the url is one of the user defined domain
If so, check if the page requested is a singular CPT and its author
is the one that saved the domain
If not, redirect the request to the original domainù

Let’s assume you save yout original domain in a constant, maybe in wp-config.php

define('ORIGINAL_DOMAIN', 'original.com');

now you can easily implement the workflow described above:

add_action('template_redirect', 'check_request_domain', 1);

function check_request_domain() {
  $domain = filter_input(INPUT_SERVER, 'HTTP_HOST', FILTER_SANITIZE_URL);
  // strip out the 'www.' part if present
  $domain = str_replace( 'www.', '', $domain);

  // if the request is from original domain do nothing
  if ( $domain === ORIGINAL_DOMAIN ) return;

  // if it is not a singular company CPT request redirect to same request
  // but on original domain
  if ( ! is_singular('company') ) {
    redirect_to_original(); // function defined below
  }

  // if we are here the request is from an user domain and for a singular company request
  // let's check if the author of the post has user meta, assuming meta key is `'domain'`
  // and the meta value is the same of domain in current url

  $meta = get_user_meta( get_queried_object()->post_author, 'domain', TRUE ); 

  if ( $meta !== $domain ) { // meta doesn't match, redirect
     redirect_to_original(); // function defined below
  } else {
    // meta match, only assuring that WordPress will not redirect canonical url
    remove_filter('template_redirect', 'redirect_canonical');
  }
}

Now let’s write a function to redirect request using current url, but with original domain

/**
 * Redirect the request to same url, but using original domain
 */
function redirect_to_original() {
  $original = untrailingslashit( home_url() ) . add_query_arg( array() );
  wp_safe_redirect( $original, 301 );
  exit();
}

The last thing to do is filter the permalink creation to use the user-defined domain for singular company CPT urls:

add_filter( 'post_type_link', 'custom_user_domain_plink', 999, 2 );

function custom_user_domain_plink( $post_link, $post ) {
  // we want change permalink only for company cpt posts
  if ( $post->post_type !== 'company' ) return $post_link;

  // has the user setted a custom domain? If not, do nothing
  $custom = get_user_meta( $post->post_author, 'domain', TRUE );
  if ( empty($custom) ) return $post_link;

  // let's replace the original domain, with the custom one, and return new value
  return str_replace( ORIGINAL_DOMAIN, $custom, $post_link);
}

At this point, you have only set DNS for your server, where all the user defined domains are aliases of the original.

Please note code is untested.

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