Lasciala perdere. il manuale dice:

'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.
Quindi non tutti i browser settano questo valore ma non solo, alcuni offrono l'opzione di modificare il valore inviato al server.