1 - "require" and "require_once" throw a fatal error if the file is not 
 existing and stop the script execution
2 - "include" and "include_once" throw a warning and the execution 
 continues 
3 - "require_once" and "include_once" as their names suggests , 
 they will not include the file if the file was already included with 
 "require", "require_once", "include" or "include_once"
try the following code: 
create a file called "index.php" 
<?php
require "first.php"; // this will include the file
include_once "first.php"; // this will not as it was included using "require"
require_once "first.php"; // this will not as it was included using "require"
?>

and another file that is called "first.php" and write the following header
-------------------------------
<h1>Hello every one</h1>
--------------------------------
i hope this will help you