Predefined variables in PHP are built-in global variables that provide useful information about the server, environment, and user inputs. These variables are automatically available without needing to declare them.
The $_SERVER variable, for example, contains details about the server and request. $_SERVER['SERVER_NAME'] returns the domain name of the server, while $_SERVER['SERVER_SOFTWARE'] provides the web server software name and version. This type of information is critical to PHP programmers to help build secure, and efficent web apps.