GPL licensed--not so useful for many purposes.
In short, using a GPL'd PHP library in your application mandates that significant swaths of your application be released under the GPL as well. That's not a problem if you wanted to release under the GPL to start with, but it's a problem for anyone else on either side of the spectrum; if you wanted to be All Rights Reserved, you couldn't, and if you wanted to be freer and license under LGPL, MIT or even public domain, you couldn't do that either.
For this reason, developers have to be quite careful about how they use GPL'd software to make sure that it doesn't impose unwanted licensing restrictions.
I respect peoples right to different on licensing choices(I'm not a huge fan of the GPL for my own stuff) but the wording of the original comment seemed a bit inflammatory in regards to the licensing which is why I questioned it.
For this simple situation:
What if I want to be freer than the GPL? What if I want to release my next project with a BSD license so that even commercial users or people who want to keep part of their source private can use it?
The GPL is not very suitable for commercial applications. How many fewer people do you think would be using GCC if not for the special exception that allows you to produce GPL-free binaries?
You can use GPL licensed software and sell it. There is nothing preventing you from commercially exploiting GPL licensed software. In fact there are plenty of companies doing so. You just have to include the source.
You might find this less attractive for your (commercial) goals and needs, but I find it quite attractive and important to have access to the source and the possibility to alter or enhance the software I use, while preventing freeloading (not quite true with GPL, see services as a loophole, AGPL is actually even better at preventing freeloading) to a certain degree.
I don't care if someone wants to use my code for commercial software. However, I don't want to give them a competitive advantage just because I wanted them to be more free. I still want to benefit from the changes they've made (if they make sense for me) in the same way they've benefited from my initial work! Now, if I have to pay for a copy of the software to get the changes (since the GPL allows for that sort of thing), so be it. I invited that by releasing my software under such a license.