How does Google know?

If a page has changed..

[phred]

Probably a really stupid question. Was noticing that last modified tags don’t seem to get sent if PHP generates the page – pages always seem to get sent status 200. So, how does the Google bot know to re-index a page? All my pages are PHP generated - should I resubmit a Google siteindex with a new date every time I change a page?

Phred

[rob7591]

The spider continuously crawls all pages and updates the content (I think).

All my pages are dynamically PHP generated and Google seems to do a great job keeping up with my updates.

Most forums and blogs are PHP too and those get updated by Google all the time.

You don't need to resubmit every time you update.