Super Globals

Name	Value
HTTP_REFERER	If the user clicked a link to get the current page, this will contain the URL of the previous page they were at, or it will be empty if the user entered the URL directly.
HTTP_USER_AGENT	The name reported by the visitor's browser
PATH_INFO	Any data passed in the URL after the script name
PHP_SELF	The name of the current script
REQUEST_METHOD	Either GET or POST
QUERY_STRING	Includes everything after the question mark in a GET request

<?php
   if (isset($_SERVER['HTTP_REFERER'])) {
      print "The page you were on previously was {$_SERVER['HTTP_REFERER']}<br />";
   } else {
      print "You didn't click any links to get here<br />";
   }
?>

<a href="refer.php">Click me!</a>