Noting a change to a static web page

How do systems that let you know when a web page has been updated work? Over the months/years have developed what started out as a quick-and-dirty has-it-gone-down check for an IT/intranet/extranet manager (suit type), into a URL/links checker, and now he wants to extend it again. On the dynamic side, the database programming has been tweaked to enable him to review material as it is being added to the content/updated. So now he wants something similar on the static pages. He's not in the same office, indeed is often away for long periods of time, so doesn't want to bureaucratically hold up all updates pending his personal review (thank goodness) - but wants to review/doesn't want to 'miss' things either... I had thought of using file system functions, eg filemtime(), but these don't work on remote calls/URLs. Then I thought that everytime a page changes, likely its size does too - but filesize() suffers the same local filesystem-only limitation. Then I noticed the new(er) functions: stat() and fstat() - the manual doesn't indicate that they suffer a URL limitation (as above). So I'll give them a try. Meantime, am I on the right approach? How do the 'commercial'/web-wide systems detect web page changes to send an 'alert' to their users/clients? Can anyone offer an improvement upon the above ideas? Please advise, =dn