If you're talking about borrowing a method or three it probably will suffice to give credit to the author, with a link to the source, in the PHPDoc block (the block comments for the method(s).
If you're wanting to take and use an entire class, classes, or full extension, while it's probably permitted under the license to smash -n- grab the code, it'd be best to contact the source author and ask / let him know that you intend to fork his work, while giving credit to him (or her) in your forked product. This is just a respect issue, and not necessarily a requirement.
I've seen some folks in this community take an extension that has really nice code, and the author deserves the accolades for, and the person forking it will make a few tiny changes, largely insignificant, and proceed to re-brand the extension as their own, along with spamming their website in all of the comments of every possible file (to drive traffic).
In short, that is really effed up. The author probably has more class than to call him out on it, but one thing is for sure, and that is the decreased willingness of that author to give more free products to the community. The fact that you're concerned about it, and wanting to do the right thing, is a good sign right from the start. Beer +1