http - How to get the previous url using PHP

Question

Suppose my site's url is given as hyperlink on some page on the internet; that page could be anything on internet - blog, orkut, yahoo, even stackoverflow etc, and someone clicks on it,and visited my site. So can we know, using php, the previous url from which the visitor came to my page?

Answer 1

Use the $_SERVER['HTTP_REFERER'] header, but bear in mind anybody can spoof it at anytime regardless of whether they clicked on a link.