Save refering keyword in PHP session

I would like to keep the keyword that customers use to visit my site in a PHP session.

Can someone explain how this is done?

So, if a client types “soccer lace” and clicks on my site in the list of results, my script will store this data in the session.

Whether it is universal or will vary depending on the search engine Google / Bing / Yahoo.

UPDATE This looks good to me: http://www.liamdelahunty.com/tips/php_google_referer.php Thanks for the input with the answers. Very much appreciated.

Many thanks