Predefined variables in PHP are special variables that are always available in all scopes throughout a PHP script. They are created and populated automatically by PHP to provide information about the server, environment, and request. Examples include $_SERVER (server and execution environment information), $_GET (HTTP GET variables), $_POST (HTTP POST variables), and $_SESSION (session variables). Pretty much these special variables save you time and effort because you don't need to create or set them up yourself which is helpful.