$_SERVER['HTTP_REFERER'] missing

From the documentation:

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.

http://php.net/manual/en/reserved.variables.server.php

When a web browser moves from one website to another and between pages of a website, it can optionally pass the URL it came from. This is called the HTTP_REFERER, So if you don't redirect from one page to another it might be missing

If the HTTP_REFERER has been set then it will be displayed. If it is not then you won't see anything. If it's not set and you have error reporting set to show notices, you'll see an error like this instead:

 Notice: Undefined index: HTTP_REFERER in /path/to/filename.php

To prevent this error when notices are on (I always develop with notices on), you can do this:

  if(isset($_SERVER['HTTP_REFERER'])) {
      echo $_SERVER['HTTP_REFERER'];
   }

OR

 echo isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : '';

It can be useful to use the HTTP_REFERER variable for logging etc purposes using the $_SERVER['HTTP_REFERER'] superglobal variable. However it is important to know it's not always set so if you program with notices on then you'll need to allow for this in your code

Referer is not a compulsory header. It may or may not be there or could be modified/fictitious. Rely on it at your own risk. Anyways, you should wrap your call so you do not get an undefined index error:

$server = isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : "";