PHP-Super global variables

Global variables should be used sparingly, since it's easy to accidentally modify a variable by mistake. This kind of error can be very difficult to locate. Additionally, when we discuss functions in detail, you'll learn that you can send in values to functions when you call them and get values returned from them when they're done. That all boils down to the fact that you really don't have to use global variables.

If you want to use a variable in a specific function without losing the value each time the function ends, but you don't want to use a global variable, you would use a static variable.

PHP uses special variables called super globals to provide information about the PHP script's environment. These variables don't need to be declared as global; they are automatically available and provide important information beyond the script's environment, such as values from a user input.

 PHP super globals

Variable array name	Contents
$GLOBALS	Contains any global variables that are accessible for the local script. The variable names are used to select which part of the array to access.
$_SERVER	Contains information about the web server environment.
$_GET	Contains information from GET requests (a form submission).
$_POST	Contains information from POST requests (another type of form submission).
$_COOKIE	Contains inform from HTTP cookies.
$_FILES	Contains information from POST file uploads.
$_ENV	Contains information about the environment (Windows or Mac).
$_REQUEST	Contains information from user inputs. These values should not be trusted.
$_SESSION	Contains information from any variables registered in a session.

An example of a super global is PHP_SELF. This variable contains the name of the running script and is part of the $_SERVER array.

<?php
echo $_SERVER["PHP_SELF"];
?> 

This variable is especially useful, as it can be used to call the current script again when processing a form. Super global variables provide a convenient way to access information about a script's environment from server settings to user inputted data. Now that you've got a handle on variables and scope, we can talk about what types of information variables hold.

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