I was going through Questions every good .Net developer should be able to answer and was highly impressed with the content and approach of this question and so in the same spirit, I am asking this question for PHP Developer.
What questions do you think should a good PHP programmer be able to respond to?
EDIT : I am marking this question as community wiki as it is not user specific and it aims to serve programming community at large.
Looking forward for some amazing responses.
NOTE : Please answer questions too as suggested in the comments so that people could learn something new too regarding the language.