require_once() will include the file once in our program.
require_once('abc.php') or die("Not Found");
This tells php that if a file which is named abc.php exists in directory then continue the program otherwise stop the execution of program.
Now if abc.php not found error message will be flashed, So you don’t have to wait for die() statement to flash error message, because here require_once() priority is higher than die() statement. System error will be flashed automatically without waiting for a die() statement to execute.