REL= ALTERNATE

I’m trying to setup a rel alternate that change dynamically for each page url.

The problem is i could use <?php get_site_url(); ?> but i use a different subdomain for it :

ca.exemple.com/fr/ and http://www.exemple.com

I have been trying to do this but it doesn’t work, anyone can help ?

<?php
$current_url = explode(“ca.exemple.com/fr/”,curPageURL());
$current_url = $current_url[1];
$current_url = “http://www.exemple.com$current_url&#8221;;
?>
<link rel=’alternate’ hreflang=’en-ca’ href='<?php $current_url; ?>’/>