Global variables are spacial type variables that can be accessed from anywhere within the Code, including inside functions, classes, and methods. 

Some global variables:

1. $_SERVER: Contains information about the server and the current request.
2. $_GET: Contains variables passed to the script via URL parameters.
3. $_POST: Contains variables passed to the script via HTTP POST method.
4. $_FILES: Contains variables related to file uploads via HTTP POST method.
5. $_COOKIE: Contains variables passed to the script via HTTP cookies.
6. $_SESSION: Contains variables accessible across multiple pages for a user's session.
7. $_REQUEST: Contains variables passed to the script via both GET, POST, and COOKIE methods.
8. $_ENV: Contains variables passed to the script via the environment.
9. $GLOBALS: An associative array containing references to all variables that are currently defined in the global scope of the script.