I looked into this more deeply, and some of our PHP repositories on GitHub show OptOutEvent and some have OptoutEvent, so that string is not consistent across our wrappers, based on which repository you download (we have a main wrapper, an email campaign example, and a contact example, among others). The casing of the string isn't very important, since it is just an arbitrary string and not a variable being called from somewhere else (that string is only declared/used once in the wrapper).
However, if you tried to use part of one wrapper and part of another, because the case does change, you could run into problems. This also would matter when parsing your results (your case would just need to match). I will look into making the wrappers more consistent; thanks again for letting us know about that.
You can post a new message in the Community or find us on Twitter Mon-Fri 8am-8pm ET, Sat 10am-8pm ET, & Sun 10am-6pm ET. We've got real people waiting to help you out. Click below to start a conversation!