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I think heuristics are much more important than knowing semantics. Someone who can memorize all the notes on a piano faster than their peer isn't necessarily going to be better at piano in the long run.

Similarly, there was an idea expressed in chess where some people rely on memory to reproduce thousands of similar scenarios, while others rely on the "feel" of the chessboard and the various heuristics related to it (e.g. is the king "safe" -- e.g. castled and not exposed etc., are the bishops "powerful and free to roam" -- e.g. not blocked by pawns or heavily contested etc.).

My argument partially comes from self-defense; my memory is comparatively poor. But I'm able to use heuristics developed over years of experience to make generally good decisions.

So in this case:

"Who would you rather hire: the person who knows exactly what features are available in PHP 7 and which are only available in PHP 8, or the one who will figure it out by trial-and-error of while writing each application and seeing what fails?"

I would hire the person who is able to write the best application, and understand when to use what features or not: similar to the Bond quote "Sometimes a trigger needs to be pulled -- Or not pulled, it's tough to know which in your pajamas". If memory was the most important part of cognitive ability computers would serve a much larger role in society than the (already large) part they do today.