What is the difference between absolute and relative file paths in PHP, and how can developers determine the correct path to include files?

Absolute file paths in PHP specify the full path to a file starting from the root directory, while relative file paths specify the path relative to the current working directory. To determine the correct path to include files, developers can use the `__DIR__` constant to get the directory of the current file and then construct the path accordingly.

// Using absolute file path
include '/path/to/file.php';

// Using relative file path
include __DIR__ . '/file.php';