The include() Function

Simplest way to include a file with PHP.

<?php include('header.php'); ?>

The require() Function

<?php require('header.php'); ?>

The require() function is identical to include(), except that it handles errors differently.

The include() function generates a warning (but the script will continue execution).  You will still see the error on the page, but after the error message, you will see all code executed after that.

The require() function generates a fatal error and the script execution will stop after the error, generating no execution of any code before or after.