Bill Kellaway on "Wildcard Subdomain - Override get_site_url()"

I have a site that uses wildcard subdomains. Enter www. or wwg. or whateve and the site works - only on the first visit. If a user visits a second page they are taken to the www version because of the URL in the menu's displaying the absolute path that is built using the get_site_url().

I am trying to add a filter to over ride the built in get_site_url() function. The filter to the last line of the functions file.

Also for this to work you need wildcard subdomains set up - which I have.

Any ideas why this isnt working ?

function set_site_subdomain() {

$server_name = $_SERVER['SERVER_NAME'] ;
$host = explode('.', $server_name );
$this_subdomain = $host[0];

$site_url_array = explode(".", get_site_url() ) ;

$new_url = "http://".$this_subdomain.".".$site_url_array[1].".".$site_url_array[2] ;

return $new_url ;

}

add_filter('get_site_url', 'set_site_subdomain', 1 );

Thanks in advance