Isn't 5 the output one would expect?

It's about the undefined execution order.

https://www.php.net/manual/zh/language.operators.precedence.php
> Operator precedence and associativity only determine how expressions are grouped, they do not specify an order of
> evaluation. PHP does not (in the general case) specify in which order an expression is evaluated and code that
> assumes a specific order of evaluation should be avoided, because the behavior can change between versions of PHP
> or depending on the surrounding code.
>
> Example #2 Undefined order of evaluation
  <?php
  $a = 1;
  echo $a + $a++; // may print either 2 or 3

  $i = 1;
  $array[$i] = $i++; // may set either index 1 or 2
  ?>

(the page is already marked as needing a translation update)

> It's about the undefined execution order.

I don't think so, since it doesn't matter whether `(++$a)` is
evaluated before `(++$a)` or the other way round.  It rather
seems to me that PHP < 7 got confused by the reference.

Sorry yes, you're right, it's a pre-increment in there, and the reference does (did) matter.

5 is the correct output - I'm not sure how it came up with 6 before.

Of course the minute after I send that I figure it out.

The reference mattered because ++$a incremented $a and then "returned" $a. In other words,
  ++$a;
  ++$a;
  echo ($a)+($a);

When PHP 7 changed references and introduced AST and redid everything else, that nuance disappeared and the expression would properly return the *value of* $a, regardless of whether the variable was a reference or not.
While I don't remember a specific example, I know there have been a few bug reports about this same change in behavior with similar situations. I might be able to find one...

Two bugs about the incorrect behavior in PHP 5,
  https://bugs.php.net/bug.php?id=71010
  https://bugs.php.net/bug.php?id=65580
both closed NAB due to undefined behavior with sequence points.

PHP 7's behavior does make more sense, but it's still technically undefined.