what is the difference between include(), include_once() and require_once()? code example

Example 1: 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';
?>

Example 2: 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';
?>