Include function is very useful when you want to add
data of one file into another file. 
Require function is also use when you want to add data
of one file into another file. Both functions are some
but there is a one difference in include and require
function is that include() function produces a warning
but the script will continue execution, while the
require() function produces a warning and a fatal error
i.e. the script will not continue execution.

There are two more functions like include_once() and
require_once() its means if you add the same file in
multiple time (Mistakenly) you file show only once. 

Include Example:
<?php 
include("safcode-header.php");
include_once("safcode-header.php");
?>

Require Example:
<?php 
require("safcode-header.php");
require_once("safcode-header.php");
?>