the first $a++ assigns 2 to $a, the second $a++ assigns 3, and then $a = 3 + 2

well, I would expect that the post increment actually postincrements:
$a++ returns $a and increments after that; so the first $a++ returns 1 and now $a is 2, the second increment returns 2 and makes $a = 3
so the addition returns 1 + 2 = 3.

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