In PHP, predefined variables are special variables that are built into the language. They are always available and give useful information about the server, the script, or the user. For example, $_SERVER['PHP_SELF'] shows the name of the file currently running, while $_SERVER['HTTP_USER_AGENT'] shows details about the web browser being used. Unlike normal variables, you do not create or set predefined variables—they are ready for you to use anytime. They help developers get important details without writing extra code.