see:
http://en.wikipedia.org/wiki/Sequence_point

Actionscript:
var i = 1;
i += ++i;
trace(i); // traces 3
//is equivalent to
var i = 1;
var oldi = i;
var x = ++i;
i = oldi + x;
trace(i); //traces 3

Php:
$i = 1;
$i += ++$i;
echo ‘i = ‘, $i, ‘<br>’; // echos 4
//is equivalent to
$i = 1;
$x = ++$i;
$i = $i + $x;
echo ‘i = ‘, $i, ‘<br>’; // echos 4

Ruby:
Ruby doesn’t support the increment operator.
So you can write ++ all you want, it will just ignore you.

Each of these approaches makes sense, they just happen to differ.

i = 1
i += i++ + ++i

first operate on the right side :

1st operation i++ => right_ans = 2
2nd operation + (i is still 1) => right_ans = 2 + 1 = 3
3rd operation ++i => right_ans = 3 + 1 = 4

then operate on the left side :

i = i + right_ans = 1 + 4 = 5

var i:Number;
i = 1;
i += i++ + ++i;
trace(“i = “+i);

i = 1;
i = i+i+(i+1);
i++;
trace(“i = “+i);

i = 1;
i = 3*i+2;
trace(“i = “+i);

In php and c++ however this would be 7 …

‘;

$i = 1;
$i++;
$i = $i + $i + $i;
$i++;
echo ‘i = ‘, $i, ”;

$i = 1;
$i = (3 * ($i + 1)) + 1;
echo ‘i = ‘, $i, ”;
?>

And in ruby it would be 3.

i = 1
i += i++ + ++i
puts ‘i = ‘ + i.to_s

i = 1
i = 3 * i
puts ‘i = ‘ + i.to_s

My head hurts.