When should one use require, require_once, include, or include_once in PHP scripts?

When including external PHP files in your scripts, you should use require or require_once when the included file is essential for the script to function correctly. This will cause a fatal error if the file cannot be included. On the other hand, you can use include or include_once when the included file is not crucial, allowing the script to continue running even if the file cannot be included.

// Using require to include an essential file
require 'essential_file.php';

// Using include to include a non-essential file
include 'optional_file.php';