Single robots for multidomains on WordPress without plugins

This article is a continuation of multisite cases on WordPress. Here is an example of replacing the standard robots.txt with a multisite one.

1. If robots.txt file has already been created, rename it or delete it.
2. In .htaccess add the line:

RewriteRule ^robots.txt$ robots.php

3. Create a robots.php file with the following content:

<?php
  header('Content-Type: text/plain; charset=utf-8');

  echo 'User-agent: *
        Disallow: /wp-admin/
        Allow: /wp-admin/admin-ajax.php
        ';

  if($_SERVER['HTTP_HOST']=='subdomain1.domain.com'){
    echo 'Host: https://subdomain1.domain.com
        Sitemap: https://subdomain1.domain.com/sitemap_index.xml';
  } 
  else if($_SERVER['HTTP_HOST']=='subdomain2.domain.com'){
    echo 'Host: https://subdomain2.domain.com
        Sitemap: https://subdomain2.domain.com/sitemap_index.xml';
  }

   else {
    echo 'Host: https://domain.com
Sitemap: https://domain.com/sitemap_index.xml';
  }
?>

Insert all the main content into the first echo, the rest of the dynamic will be placed according to the conditions with the host check.