In my time at MySQL AB in the Community Relations possition (2004-2006) I wrote several articles on MySQL’s licensing for the MySQL web site. The core reason for having to explain anything was (and still is) the dual licensing of MySQL, in particular the client library. I left MySQL AB years ago, but people still ask me licensing questions. Below is an excerpt from one such question, and my response.
> Hi, Found a post on the mysql website from Arjen Lentz to do with the whole
> mysql licensing question.
> Do you know if the issue with, php scripts (that use a mysql database) issued
> under a proprietary license require you to have a commercial license for
> mysql, or will the issues be covered for the GPL version through the fact
> that the scripts run via php which in-turn connects to the GPL mysql server
> for which the FOSS exception applies.
Note: I am not a lawyer; this is not legal advice.
The issue might be a bit fuzzy since you are actually dependent on MySQL server, whether or not you are “linking”. So the linkage could be there anyway (there’s no consensus on this interpretation of “linking”, it is however the viewpoint of some – hence the fuzzyness).
My recommendation to you would be to not fuss with any nasty licensing for the PHP code you create for clients. While this provides the client with more freedom, you are the expert and thus the first choice for any support and future development. Providing clients with freedom tends to bind them more to you, while restrictions tend to make them look around for alternatives.
Your clients are in whatever business they’re in, which is probably not PHP code development; it’s not in their interest to go spend time on that or undermining you, unless you were to provide bad service.
If you approach your software in this way with your clients, you can generally GPL it and do equal or better business while not having to worry about nettly licensing questions. You don’t want to base your business on a legal argument, as you just don’t want the question to get raised to begin with… it’d be costly and distracting (if not destructive).