What are the best practices for including files in PHP, and why should they not be included without __DIR__?

When including files in PHP, it is best practice to use the `__DIR__` constant to ensure that the file paths are relative to the current file's directory. This helps prevent issues with file inclusion paths and makes the code more portable and easier to maintain. Without using `__DIR__`, the included files may not be found or may be included from unexpected locations.

// Correct way to include files using __DIR__
include __DIR__ . '/path/to/file.php';