As MySQL's commercial licensing scheme at http://www.mysql.com/products/licensing/commercial-license.html says "Typical examples of MySQL distribution include: ... Selling software that requires customers to install MySQL themselves on their own machines."
Now whether or not that is actually MySQL distribution and whether or not one's software product is a derivative work of MySQL is still under contention, but assuming that this is true, wouldn't a website developer selling a custom PHP/MySQL-driven site need to pay for a MySQL license as well?