He is using Wordpress so I assume PHP: $_SERVER['HTTP_REFERER']; http://php.net/manual/en/reserved.variables.server.php http://php.net/manual/en/reserved.varia

He is using Wordpress so I assume PHP: $_SERVER['HTTP_REFERER']; http://php.net/manual/en/reserved.variables.server.php http://php.net/manual/en/reserved.variables.server.php HTTP_REFERER "The address of the page (if any) which referred the user agent to the current page. This is set by the user agent. Not all user agents will set this, and some provide the ability to modify HTTP_REFERER as a feature. In short, it cannot really be trusted."