Object Oriented Programming (OOP)
Initially when I started writing PHP the support for OOP was limited, in a sense its still not a full “Object Orientated” system, that said I have been coding in this way for at least 4 years, I cant really recall when I made the switch from action based coding to OOP.
Writing in OOP is pretty much the industry standard now, if a developer isn’t competent in OOP I would consider looking not working with them on any project as any plugins / APIs you integrate with will no doubt be coded in an OOP standard