PHP Superglobals
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Debugging Tip
In order to be able to debug many PHP scripts, you often need to confirm exactly what data a PHP page receives or can access. Towards this end, printing the values of a Superglobal in PHP can be quite useful.
Server Variations
Different than $_GET, $_POST, $_COOKIE, $_REQUEST, $_SESSION, and $_FILES, the values of the $_SERVER and $_ENV Superglobals will differ greatly from one server to the next. Everything from the Web server being used (Apache, IIS, etc.) to the operating system to the user that PHP runs as will play a role. Some of the $_SERVER elements may not be reliable, such as $_SERVER['HTTP_REFERER'], and practically all of $_SERVER is meaningless when running PHP from the command line.
Altering the PHP Superglobals
You cannot define your own Superglobal variables in PHP. You can, although this is not commonly necessary, add values to $GLOBALS (or even $_GET, $_POST, and the others).
The Superglobals essentially replace older versions of each array, which are now deprecated. For example, it is preferable to use $_GET instead of $HTTP_GET_VARS and $_COOKIE instead of $HTTP_COOKIE_VARS. As of PHP 5, the older versions of these variables, for example, $HTTP_GET_VARS, can be disabled.
Note that technically the new and older versions of each variable (e.g., $_GET and $HTTP_GET_VARS) are separate variables that initially store the same values. Manipulations of one array will not affect the other:
// Assume this page is accessed as page.php?var=value;
$_GET['var'] = 'new';
echo $_GET['var']; // Prints "new".
echo $HTTP_GET_VARS['var']; // Prints "value".
This same relationship exists between $_GET, $_POST, $_COOKIE, and $_REQUEST. Changes made to one Superglobal array within a script will not affect the other.
// Assume this page is accessed as page.php?var=value;
$_GET['var'] = 'new';
echo $_GET['var']; // Prints "new".
echo $_REQUEST['var']; // Prints "value".
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