Properly mocking methods allows you to get rid of this unknown and say, "The submit() method will return an array of this specified data" without Authorize.net ever getting a ping.
With static properties, the problem is that they do have memory. If you change it in one place in your codebase, that static property will then be changed across your whole codebase for that execution, even if you have multiple, separate object instances.
As far as static properties go, if you're mucking with those you're back into the world of storing state, which OO is more suitable for, but the whole point is to avoid doing that wherever possible. :) What I was getting at before is that we really use a mixture of both real OO and procedural with class-namespaces-static-methods wherever each thing makes sense, but given the nature of what most backend PHP is trying to accomplish, the latter ends up being the majority.
