$GLOBAL is a php super global variable which can be used instead of 'global' keyword to access variables from global scope, i.e. the variables which can be accessed from anywhere in a php script even within functions or methods.
$_REQUEST, $_POST, $_GET
$_FILES, $_ENV, $_COOKIE, $_SESSION
Here is an example to testify the theory mentioned above:
$s = 25;
$t = 50;
$GLOBALS['v'] = $GLOBALS['t'] - $GLOBALS['s'];
In the code above, since v is a variable present within $GLOBALS array, it is accessible form outside the function also.
View the example of $GLOBALS variable in the browser