Redirects alias site wp-admin to original site admin when using mercator plugin https://github.com/humanmade/Mercator.

mercator-https-domain-alias.php

<?php
/**
 * Plugin Name: Mercator https domain alias
 * Description: Redirects mercator aliases into main site when wp-admin is accessed
 * Version:     0.1
 * Author:      Onni Hakala / Geniem Oy
 * Author URI:  https://github.com/onnimonni
 * License:     MIT
 */

// If user wants to get into /wp-admin or /wp-login redirect them into the main domain
if ( substr( $_SERVER['REQUEST_URI'], 0, strlen( '/wp-admin' ) ) === '/wp-admin' ||
     substr( $_SERVER['REQUEST_URI'], 0, strlen( '/wp-login' ) ) === '/wp-login'
    ) {

    // Check if we are in mercator alias and redirect wp-admin and wp-login requests into real domain
    $mapping = Mercator\Mapping::get_by_domain( $_SERVER['HTTP_HOST'] );

    // If mapping was found redirect into the main domain
    if ( $mapping !== null ) {
        $redirect = get_site_url( $mapping->get_site_id(), $_SERVER['REQUEST_URI'], 'https' );

        // Add some debugging information
        header( "X-Site-Mapping: " . $mapping->get_site_id() );

        // wp_redirect can't be used yet
        header( "Location: {$redirect}" );
        http_response_code( 302 );
        die();
    }
}