The "may print either 2 or 3" issue only happens with 2 PHP versions:
* PHP 4.3.0 (released in 2002 and fixed 2 months later in 4.3.1)
* and 5.0.5 (released in 2005 and fixed 3 months later in 5.1.0).

That's why I consider this "may print either 2 or 3" sentence as an inconsistency

You're already confused about what precedence means. Precedence (and associativity) is about grouping operands and says that (eg) $a+$b*$c will be interpreted as $a+($b*$c). It does not say that $a+$b*$c will be *executed* using $b first, $c second, and $a third - or any other way.

> echo $a + $a++;
PHP does not guarantee when the $a and $a++ are evaluated. It could be that the $a comes first (=1) and the $a++ second (=1 and $a=2) resulting in 1+1=2, or it could be that $a++ comes first (=1 and $a=2) and $a second (=2) resulting in 1+2=3.

> $array[$i] = $i++;
Same situation. PHP does not guarantee when the $i and $i++ are evaluated. It could be that the $i comes first (=1) and the $i++ second (=1 and $i=2) resulting in 1=>1, or it could be that $i++ comes first (=1 and $i=2) and $i second (=2) resulting in 2=>1.

Consider:
  function f() { echo "f"; return 1; }
  function g() { echo "g"; return 2; }
  function h() { echo "h"; return 3; }
  echo f() + g() * h();
Precedence states it will be grouped as f+(g*h) and thus the result will be 1+(2*3)=7. But do you know how it will be executed? Are you sure?
Take a look: https://3v4l.org/LVr4o

Your homework is to learn about operator precedence vs. order of evaluation, and optionally what sequence points are. It's mostly about C but a lot applies to PHP too.