Patrick Louys has written up a new post to his site that gets into detail about one of the SOLID development principles - the Open/Closed Principle - and how it can be applied in PHP.

I am a big proponent of the SOLID principles. But one of the principles - the open/closed principle - is often misunderstood. [...] Bertrand Meyer stated it first in his book "Object-Oriented Software Construction" in 1988. The problem with it is that some people see the word extension and they think that it is talking about inheritance (because PHP uses the extend keyword for inheritance).

He goes on to talk about a comment from Reddit and uses it as an illustration about the "extension" misconception and the commentor advocating against dependency injection. He then gets into some code showing a "Logger" class that writes to the filesystem and trying to extend it to add functionality. He covers how using a dependency injection container can help some of the inheritance issues (using a "base" class) but ultimately steps back to provide another solution. The re-applies both the open/closed principle and dependency injection to create a system where the "base" Logger class is a dependency rather than a parent class.