WordPress at a glance
filter is not described

site_url filter-hook . WP 2.7.0

Filters the site URL.

Usage

add_filter( 'site_url', 'filter_function_name_6252', 10, 4 );
function filter_function_name_6252( $url, $path, $scheme, $blog_id ){
	// filter...

	return $url;
}
$url(string)
The complete site URL including scheme and path.
$path(string)
Path relative to the site URL. Blank string if no path is specified.
$scheme(string/null)
Scheme to give the site URL context. Accepts 'http', 'https', 'login', 'login_post', 'admin', 'relative' or null.
$blog_id(int/null)
Site ID, or null for the current site.

Where the hook is called

get_site_url()
wp-includes/link-template.php 3036
return apply_filters( 'site_url', $url, $path, $scheme, $blog_id );

Where the hook is used (in WP core)

Does not used.