If called from command line, the server variable HTTP_USER_AGENT is not set. I use this constant to define, whether the script is called from command line or not:
define("CLI", !isset($_SERVER['HTTP_USER_AGENT']));
UPDATE: Since this answer is still marked as the ‘correct’ one, I’d like to revise my statement – relying on the “User-Agent” header can be problematic, since it’s a user-defined value.
Please use php_sapi_name() == 'cli' or PHP_SAPI == 'cli', as suggested by Eugene/cam8001 in the comments.
Thanks for pointing this out!