What best practices should be followed when including PHP files in a script?

When including PHP files in a script, it is important to follow best practices to ensure security and maintainability. One common best practice is to use the `require_once` or `include_once` functions instead of `require` or `include` to prevent multiple inclusions of the same file. Additionally, it is recommended to use absolute paths when including files to avoid any path resolution issues. Lastly, it is a good practice to sanitize and validate any user input before including files to prevent any security vulnerabilities.

// Example of including a PHP file using require_once with an absolute path
require_once __DIR__ . '/path/to/include_file.php';