此代码运行后为何会输出5?

$a = 1;
$b = &$a;
$c = (++$a) + ($a++);
var_dump($c);

为何会输出5?
[解决办法]
$c = (++$a) + ($a++);
$c = (++1) + ($a++);
$c = 2 + 2++;
$c = 2 + 3;
$c = 5;