WordPress Subdomain Plugins: Posts, Pages, Categories and Authors on Their Own Subdomains

add_action( 'parse_request', function ( WP $wp ) {
    // Never touch admin, REST, cron, or the main domain root.
    if ( is_admin() || wp_doing_cron() || wp_doing_ajax() ) {
        return;
    }
    if ( defined( 'REST_REQUEST' ) && REST_REQUEST ) {
        return;
    }

    $host = strtolower( $_SERVER['HTTP_HOST'] ?? '' );
    $base = wp_parse_url( home_url(), PHP_URL_HOST );
    $base = strtolower( $base );

    if ( $host === $base || $host === 'www.' . $base ) {
        return; // main domain — let WordPress behave normally
    }

    if ( ! str_ends_with( $host, '.' . $base ) ) {
        return; // not one of ours
    }

    $sub = substr( $host, 0, -( strlen( $base ) + 1 ) );
    if ( ! $sub || str_contains( $sub, '.' ) ) {
        return; // only single-label subdomains
    }

    // Resolve the subdomain to a post, page, category, or author.
    $mapped = my_subdomain_lookup( $sub ); // custom table: subdomain -> object_id, type

    if ( ! $mapped ) {
        status_header( 404 );
        $wp->query_vars = [ 'error' => '404' ];
        return;
    }

    switch ( $mapped['type'] ) {
        case 'post':
        case 'page':
            $wp->query_vars = [ 'p' => (int) $mapped['object_id'] ];
            break;
        case 'category':
            $wp->query_vars = [ 'category_name' => $mapped['slug'] ];
            break;
        case 'author':
            $wp->query_vars = [ 'author_name' => $mapped['slug'] ];
            break;
    }

    // Preserve pagination and feeds from the original path.
    if ( preg_match( '#/page/(\d+)#', $wp->request, $m ) ) {
        $wp->query_vars['paged'] = (int) $m[1];
    }
    if ( str_contains( $wp->request, '/feed/' ) ) {
        $wp->query_vars['feed'] = 'feed';
    }
}, 10, 1 );

The early returns for admin, REST and cron are the difference between a working plugin and one that takes down the block editor.

function my_subdomain_dns_ok(): bool {
    $probe = 'dnscheck-' . wp_generate_password( 8, false ) . '.' . wp_parse_url( home_url(), PHP_URL_HOST );
    $ip    = gethostbyname( $probe );
    // gethostbyname returns the input unchanged on failure.
    return $ip !== $probe && filter_var( $ip, FILTER_VALIDATE_IP );
}

add_action( 'admin_notices', function () {
    if ( ! current_user_can( 'manage_options' ) ) {
        return;
    }
    if ( my_subdomain_dns_ok() ) {
        return;
    }
    echo '<div class="notice notice-error"><p>';
    esc_html_e( 'Wildcard DNS is not configured. Add a *.yourdomain.com A record pointing to this server before enabling subdomain URLs.', 'my-textdomain' );
    echo '</p></div>';
} );

Shipping the preflight check turned a "your plugin is broken" ticket into a clear, fixable admin message.

add_filter( 'get_canonical_url', function ( $canonical, $post ) {
    if ( ! my_is_subdomain_request() ) {
        return $canonical;
    }
    return my_subdomain_url_for_post( $post->ID );
}, 10, 2 );

Without this, every subdomain page declares the path-based URL as canonical and the subdomain never ranks.