[PHP] Better Google PageRank with PHP?

From what I understand, and correct me if I’m wrong, one of the many factors in determining a website’s rank in Google is how frequently it is updated, or the length of time since the last update, or something to do with updating the site often… or something like that.

My question is, what is considered an “update?”

Is it the last time the page was modified, or is it whether or not the page is different since a web crawler’s last visit? If it is the latter, then PHP could be used heavily to keep the site fresh. For example, the entire homepage of a site could be generated randomly. Not the layout or anything, just the data that get’s loaded in.

I’m sure I’m not the first person to have thought of this, so anyone can feel free to clue me in.