$_SERVER["PHP_SELF"] contains the path and filename of the currently running PHP script.
$_SERVER["REQUEST_METHOD"] tells PHP which HTTP request method was used to access the page, such as GET or POST.
Predefined variables are variables that PHP automatically provides to a program. They contain useful information about the server, request, environment, and other parts of the PHP program. They are useful because programmers can access this information without creating the values themselves.