An API is just as easy to make as a "normal" PHP page, the only difference is that the intended audience is a machine rather than a person browsing your site. Instead of outputting HTML, you'll just as likely be outputting XML (JSON, PHP, plain text or, yes, HTML).

I'm still not clear what you're trying to achieve. Is it just that whenever your PHP script is executed (on lots of different websites) they "call home" and update something (hit counter in DB, possibly)?