First of all we should know what is the difference and similarity between include() and require(), include() and require() are used to include other files into a PHP file, and the difference between them are the include() generates a warning when there is an error, but the script will continue execution, otherwise, the require() generates a fatal error, and the script will stop.
The require_once() statement is similar to require() except require_once() will check if the file has already been included, and if so, it will not require it again, same with include_once().