register_url is a filter applied to the URL returned by the function wp_registration_url(), which allows you to have that function direct users to a specific (different) URL for registration.
The following example would return a registration URL http://mydomain.com/register/ for the wp_registration_url() function:
add_filter( 'register_url', 'my_register_page' );
function my_register_page( $register_url ) {
return home_url( '/register/' );
}
The URL that is passed to this filter is generated by site_url using the 'login' $scheme:
site_url( 'wp-login.php?action=register', 'login' )
Since: Version 3.6
register_url is located in wp-includes/general-template.php