how to use a different domain/subdomain for authors/catagories on single site?

wp-config.php

if ( is_alt_domain( $_SERVER['SERVER_NAME'] ) ) {
    $domain = str_replace( 'www.', '', $_SERVER['SERVER_NAME'] );
    define( 'WP_SITEURL', 'http://www.' . $domain );
    define( 'WP_HOME', 'http://www.' . $domain );
} else if (is_sub_domain( $_SERVER['HTTP_HOST'] ) ) {
    $domain = "{$_SERVER['HTTP_HOST']}";
    define( 'WP_SITEURL', 'http://' . $domain );
    define( 'WP_HOME', 'http://' . $domain );
} else if (! (is_main_domain( $_SERVER['SERVER_NAME']) ) ) {
    Header( "HTTP/1.1 301 Moved Permanently" );
    Header( "Location:http://family.com", true, 301 );
    exit;
}
function is_main_domain( $domain ) {
    $domain = $_SERVER['HTTP_HOST'] ;
    return strcmp( $domain, my_main_domain() ) == 0 ;
}
function is_sub_domain ( $domain ) {
    $domain = str_replace( my_main_domain(), '', $_SERVER['HTTP_HOST'] );
    return in_array( substr( $domain, 0, -1), sub_domains() );
}
function is_alt_domain( $domain ) {
    $domain = str_replace( 'www.', '', $_SERVER['SERVER_NAME'] );
    $domain = substr( $domain, 0, strrpos( $domain, '.') );
    return array_key_exists( $domain, alt_domains() );
}
function sub_domains() {
    return array(
        'me',
        'her',
        'kid1',
    );
}
function alt_domains() {
    return array(
        'project1' => 'com',
        'project2' => 'com',
        'project3' => 'is',
    );
}
function my_main_domain() {
    return 'family.com';
}

functions.php

// this seems to be important
update_option('siteurl','http://' . $_SERVER["HTTP_HOST"]);
update_option('home','http://' . $_SERVER["HTTP_HOST"]);

add_action( 'category_link', 'yoursite_category_link', 11, 2 );
function yoursite_category_link( $category_link, $category_id ) {
    $parts = explode( "https://wordpress.stackexchange.com/", $category_link );
    // if %category% in http://%domain%/%category%/
    // is in alt_domains() update the category_link
    if ( array_key_exists("{$parts[3]}", alt_domains() ) ) {
        $tld = alt_domains()[$parts[3]];
        $category_link = "http://www.{$parts[3]}.{$tld}/"; // Strip 'category/%category%/'
    }
    return $category_link;
}

add_action( 'author_link', 'yoursite_author_link', 11, 2 );
function yoursite_author_link( $author_link, $author_id ) {
    $parts = explode( "https://wordpress.stackexchange.com/", $author_link );
    // if %author% in http://domain/author/%author%/
    // is in sub_domains() update the author_link
    if ( "{$parts[3]}" == "author" && in_array("{$parts[4]}", sub_domains() ) ) {
        $author_link = "http://{$parts[4]}.family.com/"; // Strip 'author/%author%/'
    } else {
        $author_link = "http://family.com/author/{$parts[4]}"; // Strip 'author/%author%/'
    }
    return $author_link;
}

add_action( 'init', 'yoursite_init' );
function yoursite_init() {
    // Remove the canonical redirect to the category page
    // if %category% in http://%category%.%ext%/ is a blogger category
    if ( is_alt_domain( $_SERVER['SERVER_NAME'] ) || is_sub_domain( $_SERVER['HTTP_HOST'] ) ) {
        $parts = explode( "https://wordpress.stackexchange.com/", strtolower( $_SERVER['REQUEST_URI'] ) );
        if ( (count($parts) > 1) &&  ( array_key_exists("{$parts[1]}",alt_domains() ) || ( "author" == $parts[1] && in_array("{$parts[2]}", sub_domains() ) ) ) ) {
            remove_filter( 'template_redirect', 'redirect_canonical' );
        }
    }
}

add_action( 'template_redirect', 'yoursite_template_redirect' );
function yoursite_template_redirect() {
    // Redirect any http://%domain%.%ext%/%category%/ to http://%category%.%ext%/
    // and any http://%domain%.%ext%/author/%author%/ to http://%author%.%domain%.%ext%
    $parts = explode( "https://wordpress.stackexchange.com/", strtolower( $_SERVER['REQUEST_URI'] ) );
    if ( array_key_exists( "{$parts[1]}", alt_domains()) ) {
        $tld = alt_domains()[$parts[1]];
        wp_redirect( "http://www.{$parts[1]}.{$tld}/", 301 );
        exit;
    } else if ( 'author' == $parts[1] && in_array("{$parts[2]}", sub_domains() ) ) {
        wp_redirect( "http://{$parts[2]}.famliy.com/", 301 );
        exit;
    }
}

add_action( 'request', 'yoursite_request' );
function yoursite_request($query_vars) {
    // If %category% in http://%category%.%ext%/ is a blogger category set the
    // 'category_name' query var to tell WordPress to display the category page.
    if ( is_alt_domain( $_SERVER['SERVER_NAME']) || is_sub_domain( $_SERVER['HTTP_HOST'] ) ) {
        $category_domain = str_replace( 'www.', '', $_SERVER['SERVER_NAME'] );
        $category_domain = substr( $category_domain, 0, strrpos( $category_domain, '.' ) );
        $author_name = substr( $_SERVER['HTTP_HOST'], 0, strpos( $_SERVER['HTTP_HOST'], '.' ) );
        $parts = explode( "https://wordpress.stackexchange.com/", $_SERVER['REQUEST_URI'] );
        $category_name = "{$parts[1]}";
        if ( array_key_exists($category_domain, alt_domains()) ) {
            if ( strlen( $category_name ) < 1  ) {
                $query_vars = array( 'pagename' => $category_domain.'-page' );
            } else if ( term_exists("{$category_name}", 'category') ) {
                $query_vars = array(
                    'category__and' => array(
                        get_category_by_slug($category_domain)->term_id,
                        get_category_by_slug($category_name)->term_id
                    )
                );
            }
        } else if ( in_array( $author_name, sub_domains() ) ) {
            if ( strlen( $category_name ) < 1  ) {
                $query_vars = array( 'pagename' => $author_name.'-page' );
            } else if (term_exists("{$category_name}", 'category') ) {
                $query_vars = array(
                    'category_name' => $category_name,
                    'author_name' => $author_name
                );
            }
        }
    }
    return $query_vars;
}

And, of course, you also need to point your subdomains (or *) on your nameserver to your wordpress install. You also will need to add a Virtual Host (Nginx) for any other domains you are using… and you will need to set up CORS.

As a side note, this answer used in conjuction with my next answer/question – How to have a static category/author page? – should make my intentions of having one multi-faced site a little more clear 🙂