Abigail, I think you misspoke. You are indicating that the addition
may precede the increments in your second choice.
Your earlier statement that behaviour is "undefined" is also incorrect. Only the order of evaluation
is undefined.
What is known about:
$a + $b
is that both sides will be evaluated before
the addition.
What is not defined is whether
$a or
$b will be evaluated first.
In my example it doesn't matter, but if both
terms use and alter the same
variables then there can be side effects
between the terms.