Adding subdomain to home_url for "add_menu_page"

Question

I am trying to add a Wordpress admin menu item that will point to a URL which is

http://docs.[currentdomain]

My plugin code is currently:

function user_manual() {
    add_menu_page(
        __( 'Custom Menu Title', 'textdomain' ),
        'Manual',
        'manage_options',
        'http://docs' . home_url(),
        '',
        'dashicons-book-alt',
        1
    );
}
add_action( 'admin_menu', 'user_manual' );

But what I end up with is currently: http://docshttp/www.domain.com

How can I achieve this without hardcoding the domain name in?

Answer 1

The way of your approach is arguably wrong. One shouldn't use add_menu_page for external links as that function isn't created for that purpose. Add menu page adds a page in the admin area with menu_slug as the fourth argument.

If we take a look at add_menu_page/dev ref source, we can notice that whatever the URL we pass in the function is passed through plugin_basename / dev ref which normalizes and replaces the path giving us the path relative to plugin directory.

---

Instead you can hook it to admin bar which in my opinio correct place to put external link(if this is only purpose)

function wpse243214_user_manual($wp_admin_bar) {
    $wp_admin_bar->add_menu( array(
        //'parent'    => 'wpse-parent-id', // parent if any
        'id'        => 'wpse243214_usermanual',
        'title'     => __('User Manual'),
        'href'      => 'http://docs.'.basename(esc_url(home_url())),
    ) );
}
//change priority(15) accordingly
add_action( 'admin_bar_menu', 'wpse243214_user_manual', 15 );

With this we get

Also note that I used basename for home_url otherwise we get a link with http or https scheme (http://www.example.com).