You can... But do you want to?
PHP scripts are not Java servlets. They're not always running; they start when a request commences and they finish when the request stops.
So, which functionality would you put that requires calling home? It must not be in every page, because it would significantly slow down both the client application and your servers. If you have some page that is rarely used (e.g. some configuration page) you could defer some of its functionality to your server. But even then, consider that your client may not want to run code that depends on some server of yours – you may not be able to guarantee it's always online; the client may have its server behind a firewall that doesn't allow outgoing connections, etc.
