Very strange PHP

Question

That's interesting, and I want to know why is it happening in that way. I have a code:
$a = 9; echo $a++;
It outputs 9, and then increases the value.
$a = 9; echo $a + $a + $a++;
It outputs 27, and that's right: 9 + 9 + $a++ where $a++ outputs 9 and then increases the value.
But here:
$a = 9; echo $a + $a++;
The output will be 19, why? It should be 18, shouldn't it? I don't get it

Answer 1

https://gist.github.com/nikic/6699370

Answer 2

Thanks!