How do companies such as vBulletin and WoltLab manage licenses given to users for the software they've bought? In other words, how do they know you've actually bought a license for the software and distinguish between "nulled" and "legit" software (that is, forums)? Obviously they do it somehow, otherwise the business model of distributing PHP-based forum software wouldn't bring in any revenue. Does it have to do with PHP, and can you implement a certain check within the software that communicates back to the licenser?