The $_SERVER['HTTP_REFERER'] variable is a server-side variable that contains the URL of the page that linked to the current page. However, this variable may not be present in all cases. For example, if the current page was accessed directly (typing the URL into the address bar or using a bookmark), there will be no referring page and $_SERVER['HTTP_REFERER'] will not be set. Additionally, some web browsers and privacy tools may block the sending of the Referer header, in which case $_SERVER['HTTP_REFERER'] will also not be set.

If you are trying to use $_SERVER['HTTP_REFERER'] in your PHP code and it is not available, you can check for its presence using the isset() function. For example:


if (isset($_SERVER['HTTP_REFERER'])) {
  // The HTTP_REFERER variable is set, so you can use it
  $referrer = $_SERVER['HTTP_REFERER'];
  // ...
} else {
  // The HTTP_REFERER variable is not set
  // You can use a default value or take some other action
  $referrer = "unknown";
  // ...