How can PHP include, require, include_once, and require_once functions be utilized to incorporate external scripts into a PHP project?

To incorporate external scripts into a PHP project, you can use the include, require, include_once, and require_once functions. These functions allow you to include and execute external PHP files within your current script. The difference between include and require is that require will produce a fatal error if the file is not found, while include will only produce a warning. The _once variants ensure that the file is only included once to prevent duplicate code execution.

// Using include to incorporate an external script
include 'external_script.php';

// Using require to incorporate an external script
require 'external_script.php';

// Using include_once to incorporate an external script only once
include_once 'external_script.php';

// Using require_once to incorporate an external script only once
require_once 'external_script.php';