Learn English – “As you mentioned” vs “as you said”

In England, "How do you do?" was until recently a commonplace greeting. The correct response was, "How do you do?" This may be what you're thinking of.

Both "How are you?" and "How are you doing?" should generally be taken as a question, to which the reply is often, "Fine, thanks!" or, more formally, "Very well, thank you." However, the whole thing continues to confuse even English people, let alone visitors.

Why stop with two? You can also speak of the prime reason, the central reason, the primary reason, the principle reason, the foremost reason, the predominant reason and the pre-eminent reason. Like main and chief, they have much the same meaning, but are likely to be found in different contexts. Only a corpus search can reveal what those contexts might be.