beautifull!

letter is not used?

Your solution fails on this test:
in_array(["foo bar", "foo", "bar"], ["foo", "bar"])
Sorry.

This solution allows duplicates in the answer and thus is wrong.

Yes, this solution isn't exactly best practice even if it is syntactically short.

Is it better to set health - damage to a variable to have the equation process just once? I'm still trying to understand pass-by-value vs pass-by-reference, and whether it even applies in situations like this. Obviously the gains are essentially non-existent in this exact case.

Because the second returns nil in this case:

If begin value is greater than the end, function should returns 0

How many words out there have 5000 letters?

isn't calling count on every character equal to a loop that iterates the whole array every time?

Use spoiler flag next time, please.