增量字母：奇怪的问题

I have just asked a question on SO and found out I can make use of ++ to increment letters. I have now tried this:

$last_id = get_last_id();

echo gettype($last_id); //string

echo 'L_ID ->'.$last_id.'<br />'; //AAF

$next_id = $last_id++;

echo 'N_ID ->'.$next_id.'<br />';//AAF

The following example which I was given works fine:

$x = 'AAZ';
$x++;
echo $x;//ABA

What is going on? Must be the end of the work day...

Thanks all for any help

++ is a post increment operator, thus

$next_id = $last_id++;

assigns the current value of $last_id to $next_id, and then increments it. What you want is a pre-increment

$next_id = ++$last_id;

Putting ++ after a variable will increment it when the statement it's part of completes. You're assigning to $next_id the value of $last_id before it's incremented. Instead, use ++$last_id, which increments before the value of the variable is used.