Superglobals are built-in variables that are available in every scope, so a function can read them without the global keyword.
$_GET: query string parameters from the URL
$_POST: form data from a POST request body
$_COOKIE: cookies sent by the browser
$_FILES: uploaded files
$_SESSION: data in the current session
$_SERVER: headers, paths and server details
$_ENV: environment variables
$_REQUEST: $_GET, $_POST and $_COOKIE combined by default
$GLOBALS: every global variable
Values in $_GET, $_POST and $_COOKIE come from the client, so treat them as untrusted. Prefer them over $_REQUEST, so you know where each value came from.
Rewriting in plainer words…
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