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.