为什么输出19？

Question

这个PHP代码背后的逻辑并不清楚答案19.答案如何才能成为19？逻辑是什么？

$i=5;
$i +=$i++ + ++$i;
echo $i;

Answer 1

$i=5;
$i +=$i++ + ++$i;
     ^
     Take value of $i as 5 then increment to 6
              ^
              increment value of $i from 6 to 7, and use the 7
          ^
          5 + 7 = 12
   ^ $i is already 7, because of the increments in the previous operations,
     so add the 12 we've just calculated, giving 19