Superglobals are built-in variables that are always accessible from any scope without using the global keyword. The main superglobals are:

• $_GET: Contains GET request data
• $_POST: Contains POST request data
• $_SESSION: Contains session variables
• $_COOKIE: Contains cookie values
• $_SERVER: Contains server and environment information
• $_FILES: Contains information about uploaded files
• $_ENV: Contains environment variables
• $_REQUEST: Contains combined GET, POST, and COOKIE data
• $GLOBALS: Contains all global variables