in php what is include,include_once,require and require_once ? code example

Example: require and require_once difference in php

As there is two different kind of require statements i.e. require and require_once
both are having same functionality of including file into the another.
  
Using require if the file is not misplaced or undefined then its stops the execution of the document.
<?php
 require 'requiredfile.php';
?>
  
And require_once is gets ignored if the file already imported with any other require or require_once.
<?php
 require_once 'require_oncefile.php';
?>