require vs require_once vs include code example

Example: php require vs require_once

//The require statement has two variations, require and require_once.
//The require/require_once statement is used to include file.

// Require a file to be imported or quit if it can't be found
<?php
 require 'requiredfile.php';
?>
// Require_once is ignored if the required file has already been added by any of the include statements.
<?php
 require_once 'require_oncefile.php';
?>