I have an application which records users visits. None of these visits are directly accessed, 100% of these visits are referred from another site.
I am passing $_SERVER['HTTP_REFERER'] through to the database. Approximately 35% of the logged entrees pass a referer, the rest are blank.
Is there a reason for this?
If a user visits your site directly, there is no referrer. It's also possible they have set it up so their browser never sends the referrer.
According to this answer, browsers do not necessarily send a referrer when doing a meta refresh.