What are some best practices for including external PHP files in your code?

When including external PHP files in your code, it is important to follow best practices to ensure your code remains organized and maintainable. One common approach is to use the `require_once` or `include_once` functions to include files, as they prevent the same file from being included multiple times. Additionally, it is recommended to use absolute paths or the `__DIR__` constant to specify the path to the file, rather than relying on relative paths.

// Include an external PHP file using require_once with an absolute path
require_once '/path/to/external/file.php';