<?php

$c64 = str_split(
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
);

$sortFunction = function($x, $y) {
    $x = !ctype_digit($x) << 9 | !ctype_punct($x) << 8 | ord($x);
    $y = !ctype_digit($y) << 9 | !ctype_punct($y) << 8 | ord($y);
    return $x - $y;
};

usort($c64, $sortFunction);

print_r($c64);