Case-Sensitive Alphanumeric Increment Function

/**************
This code is public domain.

This function increments a case-sensitive alphanumeric
number.  It will use as many digits as you give it.  If
it tries to increment a number that is at it's maximum
(eg "ZZ"), it returns boolean false.
**************/

define('NUMBER_START', ord('0'));
define('NUMBER_END', ord('9'));
define('LOWER_START', ord('a'));
define('LOWER_END', ord('z'));
define('UPPER_START', ord('A'));
define('UPPER_END', ord('Z'));

function incrementNumber($number) {
    if (strlen($number) == 0)
        return false;

    $number = str_split($number);
    $last = ord(array_pop($number));
    switch ($last) {
        case NUMBER_END:
            $last = LOWER_START;
            break;
        case LOWER_END:
            $last = UPPER_START;
            break;
        case UPPER_END:
            $last = NUMBER_START;
            $inc = incrementNumber(implode($number));
            if ($inc === false)
                return false;
            $number = str_split($inc);
            break;
        default:
            $last++;
    }
    $last = chr($last);
    $number[] = $last;
    return implode($number);
}

// Usage
echo incrementNumber('0000'); // outputs "0001"
echo incrementNumber('0aB4'); // outputs "0aB5"
echo incrementNumber('0aB9'); // outputs "0aBa"
echo incrementNumber('0aBz'); // outputs "0aBA"
echo incrementNumber('0aBZ'); // outputs "0aC0"