How is the connection of external scripts or files to PHP for execution carried out?

In PHP, the constructs include, require, include_once, and require_once are used to include external scripts or files.

• include 'file.php'; — includes the file, issues a warning on error, but the script continues to run.
• require 'file.php'; — includes the file, issues a fatal error on error, and stops execution.
• include_once and require_once — similar, but ensure that the file is included only once.

Example:

include 'config.php';
require_once 'functions.php';