echo $_SERVER['SERVER_ADDR'];
should work. I mean, your clients are out on the Internet somewhere right?
I would register a domain with dynamic DNS if I were you. Save you some long distance phone bills 😉
This is about the tenth time I've edited this post (might be a record)...If you're using PHP in combo with IIS, $_SERVER['SERVER_ADDR'] will return the routable ip address if you specify it in Internet Information Services. If you're using it with Apache it will return correctly if you configure the new ip in the <VirtualHost> directive.
Either way, DHCP is rough, spend like 3 bucks a month and get some webhosting ;-)