how do you detect if your website visitor came from a google search result?

Question

when a user searches from google and lands on our site from the results he/she was shown in the results page, is there a way for my site to detect that he came from google?<

Answer 1

It looks like you can use the $_SERVER['HTTP_REFERER'] variable. But I think the value is controlled by the client and can't always be guaranteed to exist or accurate.