$_ENV and $_SERVER ?

PHP sets several variables for you containing information about the server, the environment, and your visitor's request. These are stored in the superglobal arrays $_ENV and $_SERVER, but their availability depends on whether the script is being run through a web server or on the command line.

Useful preset variables in the $_SERVER superglobal

Name	Value
HTTP_REFERER	If the user clicked a link to get the current page, this will contain the URL of the previous page, or it will be empty if the user entered the URL directly.
HTTP_USER_AGENT	The name reported by the visitor's web 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. Not available on the command line.

Of those, HTTP_REFERER and HTTP_USER_AGENT are the most important, as you can use these two to find out a lot about your visitor and then take the appropriate action. For example:

<?php
            if (isset($_SERVER['HTTP_REFERER'])) {
          print "previously was {$_SERVER['HTTP_REFERER']}

      <br />";
            } else {
              print "You didn't click any links to get here<br />";
            }
    ?>

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

If you load that page in your browser by typing the URL in by hand, the "You didn't click any links to get here" text is shown because HTTP_REFERER has not been set. However, if once the page is loaded you follow the "Click me!" link, the page will reload itself; this time, HTTP_REFERER will be set and the other message should appear. Although it can be easily spoofed, HTTP_REFERER is generally a good way to make sure a visitor came from a certain pagewhether you want to use that to say, "You can't download my files because you came from another site" or "Welcome, Google users!" is up to you.