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'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.
Referrer information is wholely unreliable. Some proxies and routers strip the information from requests. Browsers can be set to not give the information out too. Javascript is not a solution either. While you may catch a few more pieces of information through it, you cannot guarantee 100%.