Explain the difference between include, include_once, require, and require_once in PHP.

include: Includes a file, continues execution if file not found (warning).\ninclude_once: Same as include but includes file only once.\nrequire: Includes a file, stops execution if file not found (fatal error).\nrequire_once: Same as require but includes file only once.\n\nUse require/require_once for critical files (like config files).\nUse include/include_once for optional files (like templates).\n\nExample:\nrequire_once 'config.php'; // Critical file\ninclude 'header.php'; // Optional template