HTTP Referer not always being passed

php referer
reefine picture reefine · Apr 13, 2011 · Viewed 19.9k times · Source

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?

Answer

Khez picture Khez · Apr 13, 2011

There are a couple of number of reasons why HTTP_REFERER might be blank.

  1. You have to understand it's an environment variable given by the browser. Meaning users can remove it or even change it, if they so intend to.
  2. Users accessing the link from a bookmark, history or by typing the link manually do not have a referer.
  3. IE has also been known to remove the referer in situations revolving around javascript. Such as window.open, window.location and even setting target="_blank" in anchors or meta refresh.
  4. Clicking an embedded link in a chat application, PDF/Word/Excel document, will also not set a referer.
  5. Using AJAX, file_get_contents, fopen and other similar functions in other languages will probably not set a referer request.
  6. cURL, fsockopen, applications that have browser-like components might not set a referer.

There are probably more situations when this could happen, I'll update if I can think of anything that seems reasonable.

Related questions

How to allow iframe embedding only for whitelisted websites?

I've a form that I'd like to embed in a website, which is on my whitelist. Other websites, that try to embed it, should get only an error page. <iframe src="https://domain.tld/getForm.php?embed=1&formId=123456"&…

Read More
PHP detect if page is reloaded with PHP_SELF

I have a form that reloads the page with the updated data: <form name="form" method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>"> ... <input type="submit" name="Submit" value="Update data"> </form> …

Read More
How do I check if a string contains a specific word?

Consider: $a = 'How are you?'; if ($a contains 'are') echo 'true'; Suppose I have the code above, what is the correct way to write the statement if ($a contains 'are')?

Read More