PHP, What did I do wrong? Simple str_reverse script.?

[Nja8wJxOP]

<?php
$string = '0123456789AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz';
$str_shuffle = str_shuffle($string);

$half = substr($str_shuffle, 0, strlen($string)/0*3-40);
$encrypt = str_reverse($half);
echo $encrypt;
?>

i want to reverse $half basically.

So instead of it shuffling the abc's and then dividing them and so forth, reverse that result.

So what would normally output:

XPfAlFGzCBNe9J1oW6SruM

outputs:
MurS6Wo1J9eNBCzGFIAfPX