(PHP 4 >= 4.1.0, PHP 5, PHP 7, PHP 8)
$_SESSION — Session variables
An associative array containing session variables available to the current script. See the Session functions documentation for more information on how this is used.
This is a 'superglobal', or automatic global, variable. This simply means that it is available in all scopes throughout a script. There is no need to do global $variable; to access it within functions or methods.